it's not like having that flashback was just an unfortunate mishap
as far as we know that is all guts knows about sex, it for sure left a scar, it makes all the sense that it would carry onto his moment with casca, unfortunately
anyway, I don't like when they do a sex scene exactly when they get together, cause all of the nuanced feelings and the sweet tenderness of guts consoling casca fade away to a random scene of them satisfying their fleshes will... but it is what it is