If this is your brother's computer that you were using during a short time, then I think it's really his business about what he chooses to look at on the web for whatever reason. If he is a sophomore in college, he is considered an adult and is probably aware of the potential dangers of meeting and/or chatting with people over the internet.

Think about why it's so important for you to tell your parents about what you saw. Are you afraid for your brother's safety? Do you have questions about homosexuality that you don't want to bring up directly with him? If your concerns are for your brother, then consider talking with him directly about what you are thinking. If you just want to spill your thoughts about what you found, maybe write your thoughts down in a journal or somewhere where your brother's feelings won't be hurt. Your brother is entitled to his privacy, and if your parents shrugged off your concerns, then perhaps they are already aware of your brother's interests and don't think it's a big deal, or don't want to deal with it at this time.

You may find the DEAL WITH IT! understanding sexual preferences resources helpful.