Depends on the styles. If you have set me up for a realistic brawl in a public setting, then you transition it into a sexfight, that would annoy me and I would hate it. However, if you set me up for a stylised private match that transitions into a sexfight, or a private sexfight that transitions into a real fight, that I could be interested in (especially if, as you say, it's done well and feels natural to the story).