The bad news: May have been a once in a lifetime opportunity.
The good news: Reality likely wouldn't have lived up to the fantasy.
Two important thing that you gain from this:
1) you where not thinking with your.....well you know what i mean.
2) Even thought having sex with two women at the same time a fantasy for most/some guys. You may be call a looser or studmuffin for not doing it, but you have more respect for your health than others would who would jump right in with that opportunity.
Just be proud that you where asked and you declined. A win/win situation.