I am a straight guy (unmarried). When I was traveling on a bus at night, while sleeping, an old man sitting near me touched my genitals. I didn't react to it. I pretended to be asleep. It was pleasurable to me so I didn't react. Did I commit a sin?
What if we changed the genders involved and said that a guy touched a woman's genitals while she was sleeping. Would that be wrong? "Now concerning the things about which you wrote, it is good for a man not to touch a woman" (I Corinthians 7:1). Sexual touching between unmarried people is wrong, regardless of the genders involved.
While you didn't initiate the contact, you did permit it to continue. In I Corinthians 6:9, in discussing various sins that can keep people out of heaven, Paul used two different terms for homosexuality. "Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals" (I Corinthians 6:9). The word "effeminate" translates the Greek word malakos. It literally means something that is soft and the Greeks used it to refer to a man who allows another man to do homosexual acts with him. The second word, "homosexuals," translates the Greek word arsenokoites. This word literally means man-bedder. The Greeks used "bed" as a euphemism for sexual intercourse. This word refers to a man who commits sexual acts on another man. While the man only touched you, he clearly was stirring up homosexual lust within himself.
How something feels does not determine right or wrong. What the man did was clearly wrong. It was also wrong that you permitted it to continue. In discussing the decay of the Greek society, Paul condemned not only those who committed crimes against God, but also those who approved of those crimes. "And although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also give hearty approval to those who practice them" (Romans 1:32). The proper response would have been to tell the man "no" firmly and loud enough that everyone on the bus knows that he is acting inappropriately. Then change seats so you are no longer near him.