one of my players did this. between sessions he used streetwise to find out if she was pregnant. then he arranged to have her killed. he showed up to the next session and I declared that his alignment had shifted to evil without telling the party why
I'd imagine it would only be really the first time but I still think it's a good train of thought. Feels like it's better to say yes with conditions rather than just outright saying no
Of course outright saying no is not a good solution but having an honest conversation with your players about the tone and purpose of the game is always beneficiary. In this example though I would still prefer "no" rather than going into the technicalities of bringing another human into the fantasy world and all its consequences because one player wanted to shit around. If this is the pitch of your game then of course you're free to do so but in this case players should already know this is allowed and intented. Most of the time it's a simple game about a group of adventurers delving into dungeons slaying evil monsters and if some want to introduce "romance" then it should either be disallowed or be in the same tone as rest of the game. YMMV of course
If it's the same XP each, then you'd still need to be a very prolific manwhore for the higher levels.
And if they start asking for financial support during pregnancy, you might need to resume adventuring just to afford the added food costs. Pregnant women get hungry.
I was in a game where a PC hooked up with an NPC and got her pregnant, resulting in a ton of problems for that character. It was quite funny when one of the other players noticed an unused sheep-skin condom listed on that character's sheet. Turns out, they'd looted a condom from a fallen baddie during a previous session but forgot about it.
They get way too into it and start keeping track of how much of their "seed they've spread" in an attempt to create a new army of children. As for child support, nobody uses their gold anyway