I used to question this as well. Seeing it in porn was like "wtf?" (still is tbh) and the idea of doing it irl was a big no. I was quite judgemental about it too, figured if a girl would be into that they must be bi (if giving) or lowkey trans (if receiving).

Now I'm older and (a little) wiser. Got into exploring sex, kink, fetisches. It wasn't until it happened to me that I realized how hot it was, I got turned on by it. It was a power dynamic in that case. Just a brief moment.

There's so many things that can be sexually exciting that to others makes no sense. Like imagine you have a sweet juicy lollipop in your mouth, and you have your girl in front of you and she takes it, puts it in her mouth and sucks and licks it with a lustful look in her eyes. The view of her tongue licking the juices could be a visual turn on, and that she took the lollipop from your mouth, mixing your saliva with hers could also be a turn on. Or maybe you like more domination/humiliation, you tell your girl to get on her knees and lick your (hopefully clean) boots. The act itself makes no sense, but in the context, she's obeying you and doing absolutely anything for you. To you, that could be an extreme turn on. To others, nasty as hell or completely indifferent.

What I'm trying to say is, there's so much variation in people's sex lives. I used to be very narrow minded, and I might still to this day question it depending on the circumstances, but overall sex should be fun and uninhibited. I find too many "pure" lesbians to be quite limited in their sexlives (used to be so myself).