Wild nephelym have two states: one where they chase you (or another neph) and one where they don't. If you see a neph you don't like running towards you, all you need to do is walk away, or lead them into tentacles, or quickly interact with a neph you do like and start having sex with them instead. In my experience, it does occasionally happen that you interact with a neph, and once you back out, they immediately switch from passive mode to active and jump you; or that, while you were interacting with a neph, another neph caught up to you, so when you back out, they immediately jump you; but it's not common enough to hinder gameplay to the degree you're talking about here, IMO.