This is a confession about something I dread. I'm in a relationship with a woman I plan on starting a family with when she's ready. But I am terrified that I might be infertile and never be able to experience the most masculine, sexy experience of my dreams since as far back as I can remember. I've held back on my passions and haven't resorted to cheating. 
I've fantasized about being with a woman who's pregnant with another man's child. For the life of me I can't understand why this turns me on so much. A single pregnant woman is vulnerable and desires to have support, love and security from a stable male figure. A man could be immersed in a world of intimacy never before experienced with another woman. For a pregnant woman to feel comforted that the thought of another man's child in her womb doesn't bother him, in fact turns him on, makes a woman's sex drive overload. If this fantasy were played out and I'm inside her, her telling me coming inside her makes the child more mine each time. This plagues my mind endlessly. 
All this makes me believe the only joyful path I can find is to be a nomad providing a stable gestation homelife where there wouldn't have been otherwise.

I won't judge what turns you on, but this is exactly why porn exists - to fulfill your inner fantasies without any real-life repercussions. I highly recommend that you simply dive into porn but don't quite recommend you let this play out in reality.
Having another man in the trio-relationship carries a lot of burden, conflict and problems, and isn't worth one good fantasy-play out. Sexual desires can be a strong force, I don't deny, but after a nice pleasure from your hypothesized scenario, comes endless problems and dysfunctions that usually plays out when you involve sharing your beloved. Much more so for the child, to have to cope with all these problems just because you wanted 'extra' pleasure. 

Regarding infertility, you can get yourself checked. If you are indeed infertile but still immensely wishes to romance intimately with a pregnant woman, then perhaps get yourself a pregnant hooker, but be prepared to discard whatever relationship you have with your significant other. You have alternatives, and it's all your choice. You can ultimately let everything play out but be ready for potential problems. OR, you could always fork out money to ask a camgirl to act out a scenario as per your requirements, as a compromise. Lust often takes away our rational thinking, which is why I'm urging you to have some.

Not everything is so black-and-white. Have you tried talking about this with your girlfriend, telling her how much you want to do stuff with a pregnant woman, and how concerned you are about infertility?

I dunno man, I've come to discover over the years that a real partner, one who loves you and wants you to be happy, will let you do what you need to do and not give you shit about it. And to be clear, I'm not talking about cheating. Cheating is when you break the rules you and your partner have established -- that's it. But if you and your partner set rules that say it's cool for you to go off and do something because you need it, that's not cheating at all. If you lock yourself into a relationship with rules that will prevent you from fulfilling something that's important to you, that's your own fault, because if you're a good person, you won't ever betray your partner's trust... which means never finding that satisfaction.

There are so many options available to you as long as your partner is cool. If she's not, though... yeah you've got a problem there.

(May 11, 2019, 5:44 pm)Akhenaten I dunno man, I've come to discover over the years that a real partner, one who loves you and wants you to be happy, will let you do what you need to do and not give you shit about it. And to be clear, I'm not talking about cheating. Cheating is when you break the rules you and your partner have established -- that's it. But if you and your partner set rules that say it's cool for you to go off and do something because you need it, that's not cheating at all. If you lock yourself into a relationship with rules that will prevent you from fulfilling something that's important to you, that's your own fault, because if you're a good person, you won't ever betray your partner's trust... which means never finding that satisfaction.

This is really terrible advice, IMO.  A “real partner” isn’t necessarily cool with their other half sleeping around, even if it’s just a one-time deal.  Someone can legitimately feel irreparably hurt by their partner/spouse sleeping with someone else, even if they consent to it, because it may affect them more than they thought and that’s no one’s fault.  Yes, there are some out there that wouldn’t have a problem with such a thing, but there are plenty that would.  And even if you are allowed to go and do something like that, what if once isn’t enough?  Are you going to go back and say you “need” this one more time? Two more times?  Five more times?

We all have fantasies, but not all of them are meant to be fulfilled.  Bamadoc, if this is truly the girl you want to settle down with, you need to be very careful how you broach this subject with her.

(May 15, 2019, 7:55 pm)doubleintegral And even if you are allowed to go and do something like that, what if once isn’t enough?  Are you going to go back and say you “need” this one more time? Two more times?  Five more times?

Then that's what you need, and you need a life partner who is OK with it -- otherwise you're just lying to yourself about who you are and how you like your relationships to operate. Awful place to find yourself in years later.

*Edit: And yes, I'm speaking from experience. Believe me, if you're genuinely curious about a lifestyle that doesn't fit the cultural norm, you need to find out if it's what you truly want, and you need to have an adult conversation with your partner about it before things get super serious and there's no wiggle room and no turning back. To ignore this is unfair both to you and your partner, and could turn you into a ticking time bomb where one day, after so many years of suppression and "what if I had..." you finally have to turn to her and have the "I'm not happy" conversation. It's a lot more difficult and terrible to have it then than it would be to have it now, so do take that under advisement. But of course, if you don't think it's worth the risk and you think everything will be fine, that this is all just a fantasy and that it will blow over and you won't have any regrets later... yes, by all means, ignore the absolute fuck out of it and don't complicate your relationship. If she's a very traditional type of person, you're kind of fucked either way with this, so choose the lesser of two evils and stay that course!

(May 15, 2019, 7:55 pm)doubleintegral A “real partner” isn’t necessarily cool with their other half sleeping around, even if it’s just a one-time deal.

You're right, that was worded very, very poorly on my part. I should have said something more general, like that a real partner, one that's good for you, will want you to be happy and will do everything they are capable of in order to support that. They may be capable of a lot, or capable of very little, but they will go as far as they personally can in support of you and your needs. I took a shortcut in my post for some reason and didn't say uhhh... any of that. My bad.

So again, it goes back to what everyone else (and I, poorly/indirectly) has been saying: figure out what you need versus what you want, how your partner does or does not fit into this, what she is and is not capable of doing (you can probably suss this out without revealing your desires, but that's your call -- I prefer full disclosure in my relationships), and so forth. But I would say that you sound concerned that something you seem to feel pretty strongly about is going to be out of your grasp if you take a certain path, and regret's a biiiiiitch... so...