No, you’re not dense at all. I was in a similar place a few years ago. Although, as a Catholic I didn’t really care if the perpetual virginity of Mary was even implied in Scripture, to me it just didn’t matter. When I was told her perpetual virginity was definitely shown in Luke, Matthew, and Numbers, I simply didn’t believe it, and besides to me it didn’t matter. I read several books which tried to explain it, still I didn’t understand. I, like you, simply didn’t get it. If you go step by step as we’re trying to, it can’t escape you. You may be reluctant because you don’t believe it will prove anything, you may be reluctant because you see exactly where it will lead, I don’t know. If you truly want to understand this Catholic teaching (and it is true) you need to go step by step. If you don’t want to understand, simply give up. It’s all up to you. You ask where I’m going with this? I’m going to show you why she must have been under a vow of virginity, I’m going to show Joseph knew of this vow when the two became engaged, I’m going to show that this teaching is not simply a mistake Catholics make, but a purposeful teaching God wants us to know (for what reason He wants I have no idea), I’m going to show that Joseph and Mary could not have had sexual relations after the birth according to Jewish Law, and I’m going to show all of this solely from Scripture. But, it’s going to take time and patience. If you’d like to learn the truth, let’s do it. If not, I understand.