Towards the beginning of the series he dropped Cyclops down a ravine in the Savage Lands (which resulted in him breaking both his legs and having to survive on bugs on several days until Magneto's guys saved him), so that he could have sex with the grieving Jean.
She was also a teenager at the time, though I think that she said that she was 19, so... marginally less creepy then what he did with MJ and Liz.
Though Jean only worked out what happened AFTER she had slept with him, so yeah, not a nice guy.