I once thought so, but now I'm not so sure. When I found what my now BF was packing, I wasn't sure quite if I could manage him but I couldn't get his 8x6" cock out of my mind. Eventually I realised that I owned a bigger dildo and I could fit that after a couple of hours play, so his cock must fit me somehow.
When we met he was "curious" and had only ever taken a dildo before, so I had to gently teach him; he learnt really well and I looked forward to him coming to me for some cock.
About 4 weeks after we first hooked up he came to me ready to be fucked and I asked him if he was interested in fucking me? He didn't answer, and when I came back from getting ready he asked me why? I lay on my back on the bed and drew him toward me; he got the message quickly and tentatively started rimming me, like I would do with him. After a little encouragement, he was doing really well and I kept him going for nearly 10 minutes; this guy was learning fast and responding well. I took his cock head in my mouth for a couple of minutes, and on one of the times when I could, asked him if he would like to fuck me. I immediately sucked his cock head in again and concentrated my tongue on his slit; it was clear he liked the idea from his fluid response.
I told him where to find condoms and lube, and to copy what I did when I fucked him; I lay back on the bed and waited. He was so gentle, lubing me deep before rubbing his big head over my arsehole; he did this several times before it dipped in and started to enter me. I encouraged him, and he entered me again and again, getting deeper each time and gently opening me up. Once he had his cockhead in me, I asked him to gently push it in and he did so ever so slowly; it was so good I started feeling faint when he was about halfway in, and I asked him to pull out and go again. He got the idea straight away and re-inserted and withdrew his cock again and again until he was balls deep; I kept thinking I was going to black out and obviously looked like it as he asked if I was OK. I nodded and asked him to continue; he resumed his gentle thrusting, full length thrusts this time. I couldn't believe how full I was, and how good it felt. I told him so, and he asked if he should keep going. Yes, please!.
I could feel tension building inside and a strong need to pee after a couple of minutes and eventually came strongly all over myself and kept dribbling. I could feel him rubbing my prostate and came again; a couple of strokes later I felt him tense up and twitch for about 20 seconds as he came. He filled the condom, and it was a beauty load. I later took him to the shower to clean up and relax, both of us were trembling for about 10 minutes afterward. I was sore the next day and needed to hurry to the toilet once, but it passed that day and I asked him later that week if he would like to have another go. He was so shy about it, but equally good the second time around and needed less instruction; some sort of instinct took over and he really go into it. When we were relaxing after, I mentioned this apparent instinct to him he shrugged and admitted he had not been really successful with young women, either being rejected or causing pain. When I asked of his interest in guys and was it new, he ashamedly admitted he had noticed my cock bulging my shorts while working on a job maybe 6 months earlier, and his interest grew to the point where he eventually tried a dildo and liked it. I had noticed his interest more recently and started teasing him during the job we had been working on, wearing boxers under my rugby shorts instead of jocks on some days and nothing on others, and his response to that was how we hooked up but it seems his interest started much earlier.