For example if two men John and Joe were french kissing and showing how they love each other with smiles, flirts, and intimate conversations you wouldn't have with your best guy friend christians would look down on, try to justify it to build acceptance, or say they are lusting (or about to lust) and are sinning since sin can be in the mind not just in behavior.
If male and female Jane and Bob did the exact same thing with the same motives without lust and thoughts of lust, christians would not see anything wrong with that. They are building a relationship and assuming they are christians (just as the men above), they will build a relationship accepted by god.
Both couples man/man and man/woman have the exact same intentions but to a christian they do not. It's not because of who they are as people but specifically what's between their legs overrides everything else.
I don't know why.