I know what Mark (and the author) meant by that "made him mine" comment, but I still had to stop and think about it when I actually read it.
Same as I also knew what Mikey meant by his "I actually felt you cum inside me" comment. Although there are no nerve endings in the walls of the colon that can detect touch, pain, or even temperature, one usually knows when his partner is actually coming inside him (unless that partner is only interested in getting his own rocks off, and has no interest in how the other person is feeling).
I can understand the comments about safe sex. As readers we have not really been informed of Mark's sexual history, and definitely not of his history as regards to being tested for STI's. Maybe it was added later, as a result if some of the comments above, but I do notice that @Arran has included a warning about unprotected sex in the endnote to this chapter. And there was at least a discussion between Mark and Mikey as to whether a condom should be used...