just stick with balls. or sac. what's wrong with clinical? it's not like you're calling them testes or testicles (which you could, btw). there's nothing wrong with calling a penis a penis or a vagina a vagina. having to make up all these new phrases and terminology is part of what makes writing smut such a sticking point for some people. just use balls. it's one word in an entire tag and i doubt anyone's going to close their laptop and be like "nope. sorry. can't ever play with you again. you used balls."