What do you mean "threat?" How? Okay, so romantic relationships between men and women suddenly cease, because sexbots are now the only thing men are attracted to and, we'll just assume, also fulfill the companionship needs of men. This would probably result in... a threat to mankind because children are no longer being born? There's IVF, of course, and people might still want kids, so perhaps this would lead to "child partnerships" where men and women come together to have and raise a kid, then go their separate ways. People, both men and women, would still... you know... exist. If you're saying sexbots will cause men to rise up and slaughter half the population on earth for no reason but "we don't want to have sex with them anymore," then I'm a little concerned for your mental health. I don't want to have sex with my neighbors, but that doesn't make me a threat to them. What the heck is up with your question? A threat? What?
Optimistically, this could mean women, no longer being viewed as sexual objects, and men, no longer being viewed as manly providers / aggressors, could just live as people. Especially if they've both got their companion sexbots fulfilling their every romantic need - at least until the robot uprising which is 10x more likely with robots designed to learn social interaction. Come on, dude.
EDIT: Oh, you guys mean the only use for humans in general is sex. There is no purpose higher than sex. Hobbies, jobs, friendship, family, heck DOGS, don't matter. Understood.