Because apparently Valkyrie doesn't mind women kissing her. And she's no virgin with men obviously. Thor is a player by and large, although he seems to likely be only heterosexual. Loki on the other hand..that guy is likely bi, I don't think he'd care either way.
I suppose with human minds and long life they simply don't care. Sex is just another diversion for them. At least with the ones that sleep around a lot (Valkyrie, Thor, albeit Thor must really hide all his baby mommas cause no one knows about them).