While I do think she fell in love pretty easy because he was the seeminly first guy to see her as a cute woman, I think that was fair and a normal way to start a romance, everything else including keeping the love would come later. But if the MC would have had sex with her and also make a drunk woman to promise him to go out with him and then push it to her after she wokes up it would have been scummy and I would have drooped it right away.
Good thing the MC later reaffirm his relationship with a sober LFM and also told her the truth(apparently).