I've wondered this myself, when faced with the harsh realities of being poly.
I love people easily, and my friends are very often attracted to me too. I don't need more than one relationship at a time, I wouldn't go looking for it...but I feel like no matter what, it's just a matter of time until I find someone I love, and they reciprocate. And when that happens, it breaks my heart to say no, because I want to love them. I don't want to reject anyone.
My mono friend who I'm not-quite-dating says that I might feel this way because I've always had a void where family love is supposed to be, and I try to fill that space with more lovers. I don't know if that's true...maybe if I have a family of my own then I won't have room in my heart for more people?
And if I don't want to spend my life with someone, but I do love them to some extent and want them to be happy...and I do couple stuff with them because that's what they want, then what does that make me? A slut, right...?
Thinking these things makes my heart heavy.