Er, getting caught up in the moment does weird things to you.
To use a personal anecdote that is similar but not quite -- I met a guy friend once in his hotel room and we started kissing. In our, uh, exuberance we somehow went from door to the floor by the TV, and I still don't remember how.
Anything can happen when blinded by love (or, uh, lust), and it's possible that Marius and Cosette were standing by the wall, perhaps to stay out of sight as much as possible, and Marius got so excited that he wanted to embrace her or something, but pulled back/pushed her away so that instead he just, I dunno, took her by the shoulders?
It's not one of those things I ever thought about much because, again, love and being caught up in the moment get you doing stuff that you later don't remember doing, or how you got to doing it.
Incidentally, why not stop by the Intro board and tell us a bit about yourself.
Obviously you have plenty to say, so not knowing what to say in your intro would certainly not apply here....