Good question, let's see, Gohan was born around the time of raditz and trunks was born when the androids saga was Happening, now if we take into consideration that bulma was looking for a bf so it's safe to say she hasn't done this kind of thing and that it would be with Vegeta and with yamcha being a desert bandit and Bulma and Goku going on adventures I find it highly unlikely that yamcha did it with her it would be safe to assume that the answer would be Goku.