Who is the rapist in the opening sequence of Dressed To Kill?

That scene wasn't actually for real, but just a sexual fantasy of her.

First lets look at the facts, Kate was actually in the shower, while her husband stood at the sink outside. So where would that mystery man raping her have come from (her husband still stood outside doing nothing)? Add to this that, as you said yourself, this isn't alluded to in any way later on. Then also consider that she was passionately touching herself right before the rape happened. And right in the middle of this supposed rape the scene cuts to Kate having quite ordinary sex with her husband in their bed.

So from this we can conclude that Kate actually fantasized about that rape. This becomes clearer when we look at her session with Dr. Elliot, where she tries to seduce him (ultimately sealing her fate this way) and later when she has this affair with the complete stranger from the museum (along with the mysterious foreplay in the halls of the museum). Kate is a sexually frustrated housewife looking for adventure. Sure, she didn't seem to enjoy this "rape", but it was in a stark contrast to her rather dull real sex life and she does secretly want such an "adventure".

(And besides that, the whole shower scenario was also in line with the millions of Psycho references featured in this homage, and the scene's switch from sensual to violent served as a fitting introduction to a DePalma movie repeatedly contrasting sex with violence, or using the former as stimulant for the latter.)