The reason is actually explained in chapter 1, the FL is the daughter of the most evil being in the previous world, and she was at least as evil as her father before the hero defeated them. Now that (supposedly) evil enemy suddenly said she want to bear his child, moreover that child will be the most powerful being due to having both hero and demon lord gene. Now put yourself in his shoes, why the hell someone that you know are evil suddenly said that they love you and want your child?
The MC has every reason in the world to protect his virginity from the FL, if anything, im glad that someone wrote a horny mc who are rational and not just a pervert who indiscriminately bang everything as long as they have hole.
For your second question, almost all hero in manga/other literatures are reluctant to kill others, some of them are even reluctant to kill monsters. What i catch from your comment is that youre annoyed that the mc is not some barbaric, inhumane person and that’s quite weird.
OK... It isn't that complicated. The child of the demon lord's daughter and the hero's will be a demon lord with the powers of the hero. That's all it is. That's everything that the dead DL said. The rest was Takahiro's imagination dialing things up to 11 and his weird misdirected sexual drive making him miss the point. I mean, the obvious solution is just to get condoms and start banging....