That isn't okay. There is nothing wrong with suggesting a threesome but if your partner isn't into it you need to let it drop. Threesomes are a pretty common fantasy, but unless you have a rock solid relationship and are poly they are rarely a good idea. If he's looking for a threesome with another woman I'd suggest asking him how he'd feel if you kept pestering him about bringing another man to bed. That might be enough to jar him to the reality of what he is actually asking. What's good for the goose is good for the gander right? But, if he's thinking about going poly and or is bi and doesn't really care about the dynamics of the threesome that isn't really going to work. Sit him down and say this is a hard limit for you and ask if he has any other fantasies that you might be into as well, or tell him one of yours. If he keeps it up after that I'd seriously consider dumping him. Fantasies are fine but trying to annoy your partner into something they are not into is wrong and shows a willingness to cross boundries.