'When you were younger, did you have this feeling deep down, as a certainty, that you were not going to get married? Like an intuition? Did you have this feeling this before or during your calling?"
Well Pal, it looks like you're stuck with us non-seminarians. But, I've been thinking about this too, off and on and for a long time. To answer your first query I would say that some do and some don't. However, IMHO, it is more a matter of a feeling of a calling and the joy of it than the feeling of "I'm not getting married, ever." Furthermore, if one cannot seem to decide between a vocation and the idea of a relationship, one belongs in the seminary. Not because you're getting away from women, no - and I know that is not what you meant - but because it is not the end once you're in. You can change your mind. Also, discernment is for all those questions too.
"Another brother has said, some would not get married even if it was optional for the priesthood and that can also be the case. The celibacy part of this and its acceptance comes as a bonus, or it can if it is not there, after one has the other aspects. And, of course, it is a sacrifice for most if not all. Mysticism, contemplation, studies, works, liturgy, prayer, brotherhood, etc., all take the place of marriage and/or a relationship with a woman.
All kinds of college degrees are indeed usefull for the priesthood. However, although you can major in what ever you want, you're going to have to have between 20+ and 30+ credits worth of philosophy; the exact number now escapes me. I know of a seminary that it is 35 credits. But, it varies or it can. If you do take philosophy outside of a seminary, make sure you stick to these courses only: Intro., Ancient phil., Medieval phil., Modern phil. and Contemporary phil.. All the rest, such as: Epistemology, Aquinas, Ethics, Metaphysics, Anthopology of Philosophy, have to be taken at the seminary.