Because, H.E., if one happens to be a Christian, the Bible's take on various subjects is inherently relevant. Also, rigorous hermeneutics/textual analysis involves a tad more than simple "selective reading." And theology, at its core, is simply the application of intellect to religious issues—in that sense, no religious person ought to be a non-theologian!
But of course. My question is not why do Christians use the Bible, but rather why we give it any credence as a moral guide. It's unusually common for people who clearly don't follow Biblical morality - whether they be atheists or even just moderate theists - to attempt to affirm that the Bible actually supports their position. That, to me, is just missing the point. With all the violence that is committed in the name of or directly by Yahweh within the Bible, as well as the vastly disproportionate punishments prescribed for what were sometimes the most meager of offenses, there are surely more obvious reasons to dismiss a certain verse of Leviticus beyond the fact that you interpret its meaning differently.
I understand that I'm starting to get outside the topic of homosexuality and more into religion in general, but it's difficult for me to talk about this honestly without running into this particular elephant in the room. And yes, in that sense you could call all theists theologians. Generally though, I've only seen the term used to describe those who study their faith significantly more than the average layman.
(Not to mention that non-religious folks often ask questions of us whose answers require theological reasoning—would a dumb stare be preferable?)
Keep in mind that I am only questioning the morality within Christian theology (as well as other religions, but that is less relevant), because morality is something that can be talked about outside the context of religion. There are things which are a matter of faith which, although I disagree with them, I tend not to bring up because they never go anywhere beyond "I believe in X" and "Well, I don't".
With that said, I've come to agree that our Book shouldn't be the basis by which everyone is forced to live. They should live by choice, or not at all.
Sounds good to me, then.