I let another man touch me in the night time. I do not know what to do.
The question is too vague for me to answer well. By "touch," I'm assuming he touched your genitals in some way, but I don't know if you mean purposely or accidentally. By "let," I don't know if you mean that you participated or didn't take immediate action to get the man away.
In other words, you could be talking about sleeping in the same bed as your brother and he bumped up against you in the night or you could be saying that someone gave you a handjob in the middle of the night. Without knowing what happened and possibly why I would have to cover all the possibilities I can think of, and I fear that I might miss the one you had in mind. So if you would like to explain what is going on, I'll try my best to show you what the Bible says you need to do.
It all happened so fast, but he put his hands down my pants and touched me. This continued for about 20 seconds, then he stopped. I feel ashamed more than anything, although I also found it a bit nice. Am I gay? Is gay really wrong?
Let's start with some basics. Your body was designed by God with basic desires. When those desires are satisfied, you get pleasurable feelings. So when you are thirsty and you take a drink, your body responds with pleasure. Your body doesn't know or care where the drink came from, it just knows that a basic desire is being met.
During adolescence, your body gained the desire and ability to have sex. It responds to sexual stimulation regardless of where that stimulation comes from. The fact that your penis got erect because someone touched your genitals simply means your body is working as designed. Your body doesn't care whether the touch came from yourself, some woman or some man. Therefore, just because a man touched your genitals and your body responded, it doesn't mean you are gay.
What this event does say is that the other man wanted to engage in homosexual acts with you. Unfortunately, this is a common way homosexuals "recruit." They will select young men who aren't experienced with sex. They violate propriety by purposely touching the areas of a person's body that respond sexually and then let the person's inexperience create doubts in their mind regarding whether they are homosexual. They might even tell the person, "You enjoyed it, didn't you?" As if that proves their point. The person is too inexperienced to realize it is no proof.
What it comes down to is that you were sexually assaulted by a guy. He committed a crime. The sin was on his part. Just like a woman who is a victim of rape, you can't accept blame for what someone else did to you.