I would say that merely being sexually attracted to members of the same sex is a temptation, not a sin. It leads to sin when one acts on the temptation through lust and/or physical acts.
But I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart.
Matthew 5:28 (New American Standard Version, 1995)
No... Christ says that merely looking at a woman and experiencing desire is equivalent to committing adultery with her - if not de facto, then at least in "one's heart."
Thus, it is not necessary to commit any overt act - experiencing "lust" (sexual desire) is sufficient to speak of "sin."
Regards,