I think it was more cluelessness, rather than a lack of chemistry.
Neither of them knew how to properly do anal sex at the time in the past, and I don't think they used any lube. It is very easy for anal to end up painful; the tissues are more delicate and tear easier. Although it very rarely happens between consenting partners, it is even possible to puncture the colon, requiring immediate surgery.
That being said, the continuation in my head has them switching. :)
that didnt cross my mind that they didnt use lube. 0_0 they were young so that makes sense.
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The seme clearly didn't enjoy their first time. Why is he pursuing a guy if neither of them enjoyed it. They clearly didn't have chemistry in the past. Magically now they do?
A role reversal would have been far more interesting.