The answer to that is probably just "attachment disorder'. There's nothing wrong with stable relationships, and the way you describe your preferences sounds completely healthy to me. It definitely does happen in practice, even if it's not the default, so don't give up!

(I post not-a-mod comments semi-regularly, but they drown in the ocean of "if you'd please review https://news.ycombinator.com/newsguidelines.html and stick to the rules when posting here, we'd sure appreciate it ̶g̶r̶r̶r̶r̶r̶r̶r̶r̶r̶r̶r̶r̶̶".)