It's not meddling with his life if you have your own boundaries. If I say I won't date someone if he moves to Japan, for instance, it wouldn't be me trying to tell him where to live - just communicating what type of relationship I am willing to be in. (That is, not super long distance.) Similarly, if you chose to decline a date because someone is married and his wife doesn't know, that isn't trying to tell him how to run his life either. It's just drawing your own lines.
Why, then, should you have any less right to draw that line when the relationship is already in progress? I bet he didn't ask when you two started going out, "hey, is it cool if I date other people and don't tell them about you?" And get a positive response. And just like nobody reasonable would think it was bossy or manipulative if your boyfirend moved to japan and you refused to go with him or carry on a long distance relationship, nobody reasonable would think it was too much to say "I am not comfortable with these other women you are dating who don't know about us. I am not going to be your dirty little secret. Come clean to them, or break up with me." Of course, you may lose the guy.