i had this kind of relationship for awhile when i was like, 16. i didn't consider myself as having a big dick then, i guess i was still growing. me and her slept with two other people somewhat regularly, a guy and a girl (not at the same time). but my gf at the time made it easy for us to be this open. the girl i'm with now, 8 years later, is much more judgmental. she wouldn't be interested in exploring sexual desires this way. which i guess i'm fine with, i don't feel that desire as strongly anymore. but, not having this option doesn't allow me an outlet for this insecurity... somehow it has happened that i go back and forth with feelings of insecurity about my size. sometimes i feel large in her, other times i don't. the quality of the sex depends almost entirely on this, it seems. lately, she has come to notice the swings in quality, and i'm not sure how to play this out. in a similar vain, if i'm touching her or going down on her, and she doesn't appear as turned on or responsive as she does during "good sex", i feel crushed and do my best not to withdraw. which, if during sex the primary goal is to not withdraw completely, then it's already going terribly and getting back into it is very difficult.