Update Is he trying to cheat or just be a good friend? (sperm donation)

My fiancé has this friend who I sort of know through him and I’ve only met her about twice. She’s pretty but was quiet around people she didn’t know that well. He often spends time with her and texts with her as he says they’re really good friends. I never saw that much of a problem just because I know he has a lot of friends which includes some girls. He recently told me that she wants her own child but is unable to find a suitable guy to have a child with and doesn’t have enough money for a registered sperm donor so she and him came to the conclusion to have one together. But to do it the old fashioned way, like have sex and conceive one, for free obviously. I asked him why can’t he just help pay for her to get a sperm donation if the idea is to help her have a child. He said children should always know their fathers, regardless if the parents are together or not, and that’s why he wants to be it, and that besides, shes his good friend so he wants to help and she really wants it. I only told my best friend about this and she said he’s obviously cheating with her and lying to me about all this. My fiancé hasnt given me any reason to distrust him, I’ve always given him my full trust through anything and he’s never betrayed it. He says he’s not lying, that he has no feelings toward her so it wouldn’t be like he was enjoying it or anything, it would simply be for the procreation and he said we could have one at the same time for us and our marriage. Idk if I’m overthinking it but I just feel uncomfortable about it esp since she’s so pretty and he’s always around her which may sound childish since we’re in our early twenties and he’s with me and is attracted to me but obviously I wouldn’t want him with a ridiculously pretty woman that he gets along with even if it’s just for 5 min lol. Much less share a child with her. I may be wrong for assuming things when he hasn’t given me any reason to but idk

Well here’s the update.

Tl:dr we broke up.

Thank you to everyone who responded. I really appreciate the positive comments and even the negative ones, just for the time you gave out. The positive ones really have been keeping me going. I hadn’t posted an update because I’ve been upset about the situation but I brought it up to him after reading some comments and asked him is he cheating on me and has she gotten pregnant. He flipped out on me and said why the hell would I think that, why I don’t trust him. we got in a full blown argument and then he suggested we “take a break”. I honestly deep down do think that he’s cheating and she’s pregnant so I’ve been mad and crying but I’m at my sister’s place, not talking to him rn and she’s giving me advice and keeping me company. I guess I was just hoping it wasn’t true because I just love him a lot. I think obviously we’re done but this does still hurt so bad. But thanks to the positive comments I do think I deserve more. Might keep you guys updated some more