We cannot help looking once, for we do not know, but can help looking twice. Now that you know the type of website this is, avoid it if it is an occasion of sin.
However, the comment is vague. What makes you think you were looking lustfully? Just because someone is scantily clothed, and you happen to see it, doesn’t mean you were looking with lust. For that to happen, it involves an interior state of lust. I can’t tell you how you were on the inside.
But if you quickly realized that you shouldn’t be looking and stopped, that doesn’t sound grave to me, if I understand everything correctly (I may not). However, if you looked and continued to look, knowing that this had a decent chance of eliciting lust in you, then this could maybe be an occasion of sin, though I dunno the gravity of such a specific offense I’d say probably venial if you stopped right after realizing you probably shouldn’t be there.
Ultimately a lot depends on your inside state (which we online may not know too well).