Apart from the "Milf" comment I don't see much in your description of events that would qualify as sexual harassment (at least not from my European view of things). Certainly unprofessional but not quite sexually harassing. As far as I understand, the transgressions were "only" verbal and there was no other inappropriate behaviour.
Here's what I would do: Collect all the facts and evidence about the BD person's inappropriate behaviour. This may involve talking to your employee about this again. Then take some quiet hours in the evening to review all of the facts. If, and only if, you come to the conclusion that the BD person's behaviour clearly qualifies as sexual harassment, get lawyers involved (and probably eventually fire).
If on the other hand you come to the conclusion that the comments were inappropriate but not sexually harassing, have a private and very serious and straight talk with your BD person about the events and that there is 0 tolerance for that behaviour, then move on. And of course fire the guy immediately if it ever happens again.