two things

One, that drunkenness directly diminishes your abilities for the video games, chess, checkers and darts. However, for many people and short of unconsciousness, it increases the abilities for sex.

Two, the relationship between the psychological effects of alcohol and the social expectations of the interaction. Alcohol is generally a nervous system depressant, but depresses different areas at different doses. The first to be depressed are inhibitions. In turn, inhibitions are social and cultural, and one of them in Modern Western societies is sex. Therefore, the psychological effects of alcohol combined with a Modern Western culture make explicitly likelier that she will have sex with you, and given you know this beforehand, comes close to physically taking advantage. This is a larger issue than with other nervous system depressants, many of which might render her physically unable or less able to resist, but not psychologically predisposed to lower her resistance. Related to alcohol's liberating qualities, there is an expectation that you shouldn't get drunk "and do something you'll regret". The "something you'll regret" might be anything in particular; but the Modern Western society easily focuses on having sex while drunk, even if it was consensual at the time; and is thus likely to categorise the event (whether or not it is remembered the next day) as one person taking advantage of the other person. While a risk in every age-disparate relationship, I don't think you need to increase that risk for free.

However, I have said and continue believing that it's different to make a previous pact "let's get drunk and have sex while drunk" than to get drunk and then introduce the sex question, because of all of the above.