depending on what you really consider the first boss then yes or no, if you count Esmarelda as a boss then no, there is no way to beat her on first playthrough, but loosing to her wont give slut lvl's. The goblin boss that would be the first actual meaningful boss, totally doable, heck I would say boss 2 (slimegirl) is probably harder to beat first playthrough than the goblins boss. Alternatively ofc you can also reset your sex stats on a newgame+ making it much easier, but it is totally doable on first playthrough to stay pure. Not to mention you HAVE to beat the bosses
to progress the game, and getting defeated does not give you any direct power. Sure it is it probably not easy to do it first playthrough but it is totally doable