It's not unusual for alcohol to accompany sex. This is particularly true in the opening stage of a sexual relationship.
Alcohol helps dissolve inhibitions.
It is often a feature of sexual communication. A man and a woman who agree to share alcoholic beverages during an evening spent alone together are very likely hinting their sexual attraction towards one another. Surely, no sensible woman would agree to be alone with a man late at night and share drinks with him if she found him unattractive. It's not uncommon for a couple to have a few drinks together, loosen up their inhibitions, and have sex. The next morning, the woman may wake up and have a few regrets about it.