Leading up to my sexual debut, I wasn't at all confident (I think few people are). My girlfriend at the time wasn't a virgin, and that kinda scared me (just as it would have scared me if she had been a virgin, I think every scenario would have been scary), and I was definitely thinking about how I would compare to the other guy she'd been with and thinking she would expect more than a first timer would be able to deliver. But I don't remember ever thinking about penis size in relation to this. I was more worried that I actually wouldn't know how the hell to have sex when it came to it and that I'd be done way too soon. Basically that I'd suck ass. Figuratively, that is... And I did. I was a nervous and blindly fumbling fool. The nervousness probably didn't help. Then we started having sex regularly and it gradually became better without ever rising above average. Then we parted and I've never really had insecurity issues when it comes to sexual intercourse since.
Then when I was 18, I met my current girlfriend, who I fell madly in love with. Thinking back, I actually do remember being extremely nervous the first time we had sex. Again nothing to do with penis size, but I desperately wanted it to be amazing. I wanted us to just instantly fit, I wanted it to somehow resemble the way I felt about her. Again, it wasn't anything special at all. But we're still together, and by now I know exactly which buttons to push. If all goes according to plan, I will never be with any other women, and in that case I can already declare that I've lived and will live a life without penis size ever having any greater influence on how I perceive sex and relationships.
So for the tl;dr version: the size of my penis hasn't yet had any influence on how I perceive sex and relationships.