Interesting that you consider your gym to be your church where you receive preaching, but don't understand why religion always comes into the fray. Maybe you meant to give another meaning?
Anyway, your religion is your concern and your beliefs are yours to be right or wrong about, as with all of us. But I can help you with what reasons Christians should be giving as to why lesbianism or homosexuality are sin. There are various places in the Old Testament, that talk about sodomy as sin, such as Deuteronomy 23:17,
or in Leviticus 18:22
and in the New Testament, in the 1st chapter of the book of Romans in the Bible.
You may want to do a search on
BibleGateway.com: A searchable online Bible in over 100 versions and 50 languages. for 'sodomy' and 'sodomites' as well as reading about Sodom and Lot. Then read the whole of the 1st chapter of Romans for context. But basically, neither women nor men should have same-sex intercourse, that being against nature.
You may believe in the Bible or not, or practice Christianity or not, but the Bible does show it to be sin and abomination. So when people you talk to say it is wrong, but don't know where in the Bible it says so, you can show them something they don't know.