You've already answered your question in your own post, because this is exactly what it was. I suggest learning from your experience, and ceasing to consume poisons which make the both of you look and act very stupid.
Alcohol does make people different. If you don't like the way you or your boyfriend act while under the influence, it would be a prudent choice to never be under the influence.
It's not rocket science. You were both drunk. He was horny, and you were insecure. The outcome of this formula was buttsex. I'm not particularly surprised at such a result.
Really, I've seen drunk people do much more stupid things.