What everyone around you tells you and what the bible says about it can be two different things. If you know it's said otherwise within the bible than people who tell you it's okay then yes, it is a sin.
I will kindly suggest with what ever sin you're feeling you're committing and same goes for any topic you have that's a concern, I'd say use google and put something like this on the search: what does the bible verses say about -insert topic here-?
It'll likely display results that will lead straight to verses.
I'll recommend this site which is often one of the first results given anyhow:
http://www.openbible.info/topics/ Hope that can answer many of your questions...