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