I admit, I can't think of a non-confrontational way to ask, "Why the hell didn't you invite my boyfriend?" I agree with lhn that an intermediary of some sort is your best bet, if you can find one. I really hope it was just a dumb mistake, because I can't think of another explanation that isn't insulting to both of you.

However, plenty of couples use formulations like "and guest" to avoid having to go through the trouble of learning who everyone's s.o.'s are.

One of my cousins got married the year before we did, and lhn was "And Guest" on the invitation. This didn't bother me too much, since we had been dating for less than a year at the time. However, my sister's invitation was addressed to (and I am not making this up) "Miss C--- and Fiance". This wasn't some quickie engagement, either--they had been going together for years and were just waiting until my (now) brother-in-law finished medical school to get married. My aunt had met him literally dozens of times!

Luckily, G. just thought it was funny. He pointed out that if my aunts were allowed to follow their natural inclinations, all of the people who had married into the family, including their own spouses, would be referred to on all invitations as "And Husband" or "And Wife". (My mom has seven brothers and sisters, and the family is just a little clannish.)