I think having a 'relationship' with the other person/s actually enhances the experience.
Once you get to know someone you learn to trust them and therefore feel more comfortable exploring other avenues rather than 'straightforward' sex
You get chance to explore some of the more kinkier sides of sex that you wouldn't necessarily get the opportunity to try with a regular partner, they may not enjoy it, like it etc.
When you share your body with someone else I want to trust that person not to hurt or laugh at me, to understand my body language, to stop when it goes to far or do it more if I like it. I dislike having to explain everything all the time.
If I wanted a quick shag I can get that off my vibrator/dildo and I can do that in a lot more places than I can having sex.
I'm not saying I won't or haven't had the one off shag, but for me its second best.