Sexual assault is a very broad charge, it could mean anything from touching someone's butt to attempted ****. There are dozens of things he could have [allegedly] done to her, to speculate which one it was is pointless to do at the moment until we get more facts.
**** has a burden of proof attached to it (ie, you need to convince a judge you have evidence when you charge the defendant). If there was any physical evidence of forced intercourse, that's what he would have been charged with.
Since he wasn't, this suggests to me that this is likely a case of he-said-she-said. Which is likely going to be a lot messier in terms of a trial.
That doesn't mean it's okay. That doesn't mean he's innocent, or that sexual assault is acceptable behavior. But I just wanted to clarify. **** and sexual assault are very different charges. If it was ****, they would have charged him with that, not the lesser offense.