At first glance, many people might say, "Of course it's easier to have sex with someone you love." I don't know about that.
I think that, particularly for guys as they go through the getting-to-know-you ritual tat accompanies "civilian" relationships, some thoughts and desires are kept hidden. Simply put, though we want to fuck her, we don't want her to know how much of a perv we are, 'cause then she might not want to fuck us, let alone, if you're dating, marry us -- "You want me to put my mouth around THAT?"; "You want to cum WHERE?"; "You want to put it WHERE?"
And then, once the relationship gets going, she's developed a set of expectations about us, so it's too late to let her see the perv lurking within.
On the other hand, no such expectations exist with someone you're not interested in continuing a social relationship with. You can tell her where you want to put it and what to do when you cum, and not risk any recriminations beyond hearing her say "no," (I still gotta get past that part) and not being able to see her again.
Is it easier to unleash your inner per/inner whore in a P4P encounter? I think so.