Snape you know what, I think PM does love/respect you, because if he didn't, he would be taking advantage of the fact you love him and jumping your bones, he would be taking complete advantage of you, because men do that, even a whiff of vulnerability in a woman and they are there like a shot. But not PM.
I don't know of any men who spend time around a woman and her children, in such a lovely, familiar, family way, when there is nothing in it for them. He is getting something out of being around you. He is getting some of his needs met, at least.
Maybe the reason he doesn't want to become fully involved with you is because he knows what he is like, or perhaps he likes the feeling of freedom with the situation as it is, I mean, if he isn't bothered about the sexual side of things he is getting his needs met. When you describe your 'sessions' they sound lovely, I feel quite envious. I know you would like more but 'more' equals sex, you seem to have everything else with PM barring the sex. A lot of romantically involved couples don't have the closeness and ease of relationship that you have with PM.
And talking about rejection, PM seems to be the only 'constant' in your life, as far as men go. You do have your times of space from each other if and when it's ever needed, but you slip straight back into the ease of your relationship when you come back together again. He doesn't appear to have ever rejected you as a person, sexually maybe, because he doesn't want to lose or change what you two have together, but he is always there, even when he's not with you in body, he is there. Maybe you subconsciously like being with PM because he is emotionally available, because it feels comfortable to you. If you were in a sexual relationship with him, it would change the dynamics and perhaps he wouldn't be as appealing to you after a while.
Even the most seemingly perfect of relationships don't have it all, and a lot don't have what you have with PM. I also think it will take someone very very special to win your heart away from PM 
But you know, I could be talking complete bollox, it has been known 