Really, I don't have a very good grasp on why I'm turned on by the things I'm turned on by, and most of my theories about it are probably wrong. But still, on this one, a couple of points:
1) I really like pussies. For some reason, I've come to associate them with sex. Getting up close and personal with one is sexy; it's a big turn on for me.
2) The biggest turn on for me is being with someone I find immensely attractive who can convince me she's totally into me. Skilled SPs will, of course, do a very good job of making me feel like they really enjoy being with me. However, they do a better job if they really are having a better time, and I have an easier time suspending my disbelief and buying into the fantasy if I've been doing things to help them enjoy themselves. DATY contributes to the fantasy in both ways.
And I wish people who obviously have never bothered to educate themselves on the topic wouldn't spout off on safety all the time, but since the topic came up, as everyone should know, STDs are far less likely to be transmitted by oral sex than by genital to genital contact (HIV in particular seems unable to survive in adult saliva). So asking why someone who always wears a condom when they put their dick there would stick their tongue there is like asking why somebody who wouldn't cross a busy highway on foot is willing to calmly cross a quiet residential street on foot. The risks aren't remotely comparable.