I've said this so many times but you shouldn't choose your partner by their MBTI.
This. So much.
That should be all there is to say, but to make it clear, asking
is looking at this the entirely wrong way, because selfishness is about as much related to types as intelligence, and all and any responses here will be only anecdotal on top of that, so it would be inconclusive even if there was a relation.
I can tell you how I behave, in case you are interested, but that's all. It's also going to be very tautological: I care about people and things I care about, and I don't care about people and things I don't care about. And I put things and people I care about before people and things I don't care about.
I care about myself a lot. Consequentially, I put myself before a lot of stuff. If I care as much or more about you, you end up on the top of the list as well. Simple logic.
So I dunno that it makes me selfish or more selfish than most people. The difference maybe is that I'm fine with that, I'm not hiding it or trying to pretend I'm different to what I actually am. That's a thing that does fit for a lot of ISTPs: What you see is what you get. In particular, concerning relationships, yes, right now, it would have to be on my terms. I have no interest in changing my life to fit a partner, currently; any potential one would be treated as a hobby -- something to fill my spare time.
But the crucial points are that 1) I would tell you this right from the start, and then it's up to you to agree (perhaps you want the same?) or walk away, with no grudges held on my part; and 2) this is true for me and my current life only. Who knows what other people are currently looking for? This can't possibly be extrapolated to anywhere. I will only add that I can't ever see myself staying in a long-term relationship that makes me unhappy. My happiness > your happiness
then. But on the flip side, every long-term relationship will be
very much a rational decision -- a step I took after much consideration, to prevent precisely this; to have someone around I can tolerate even if no feelings are involved.
Edit:
But I do know this- ISTPs WEAKEST function is feeling, and there feeling is usually the equivalent of a 10 year olds mentality.
I feel vaguely amused and/or offended XD
What kind of feeling does a 10 year old have, anyway?