Well for one thing, being under the influence of alcohol can sometimes cause problems for males to get it up or keep it up. A balanced measure of alcohol consumption is required for best effect while having sex under the influence.
There is also the risk of knowing when to cut yourself off, as some people tend to keep going until they puke. Now honestly, who wants to have sex with someone who smells like vomit? (Sorry, even when I'm drunk I can still smell)
Then there's the other aspect where some people go overboard, hit the bars or house party and then before they know it, they're waking up in a bed with 18 other people whom are all equally naked and sticky like yourself from all the drunken and forgoten sex the night before.
Time to go see the doctor for a checkup.
(Bubble burst)
Smoke weed and have sex..... it's just better