That's one of the reasons but there are others. Plus, there are two types of annulments -
the legal type and also one in the Catholic church.
I was actually a witness in an annulment for a friend of mine. The couple had divorced and he, my friend, wanted an annulment so he could remarry in the Catholic church. I had to find a nice way to say the woman was bat**** crazy (I TOLD him not to marry that loony-tune) but it was that she told me she never wanted children that sealed the deal for the annulment. Messy business all around, though.