To elaborate, you say you receive great pleasure from doing it. Well, there you go, that's why you do it, it gives you pleasure to pleasure someone else. You see that vag or dick and your neurons start firing away like "if i suck or lick this, i will receive pleasure" so you do so. You have probably at some point seen a piece of food that looked gross but gave it a try anyway, just like with oral, but it turned out to be an unpleasureable experience, so from then on, your brain says "this isn't going to give me pleasure" so you dont eat it. Whereas when you starting giving oral, you became aroused and your brain and body received pleasure from it, thus conditioning you to like it. And then there are people who do it because they think the other person will do it to them. They may find the act gross, but also think that pleasure will come afterwards. Think back to Pavlov