Hi lgbg, there have been so many comments made that I don’t know if anyone made this specific point. Other than having agreed to marry each other sometime in the future, a chosson and kallah have no halachic relationship. They are not permitted to have relations just like they were not permitted before they became engaged. Again, the engagement is halachically meaningless. So the kallah is an ervah to the chosson and any other man because she has not gone to the mikvah. A niddah is one of the arayot, although a slightly atypical one because most arayot are within a specific family.
The tricky part here is that there is an emotional bond between them (I hope) while the physical bond must wait till the wedding. So the degree to which they should interact, see each other, spend time together, etc…is not simple, and as usual, there are many different approaches, and each couple should follow their halachic and spiritual authority.