Most gay people, once they've come out, aren't struggling with homosexuality. They've accepted it as part of who they are. They may be struggling to understand how that impacts their life now, how to go forward in honesty and integrity as a gay man or lesbian. They may be struggling a lot with rejection and condemnation piled on them because of who they are. They struggle to find a place in a society that assumes heterosexuality. But they aren't struggling with their sexuality.
What people are actually implying with the phrase is that they believe the person is struggling with the consequences of some unwise choices they are assumed to have made in their actual sex lives. The assumption is that anyone who comes out as a homosexual has had sex because, hey, the word "sex" is right there in the word. Sometimes we give them the benefit of the doubt and figure they've come out as a sort of early warning of impending sexual relations, a sort of "I'm telling you this now because I'm planning to go out and get a boyfriend/girlfriend and have lots of sex" thing. Given that teens and even children are coming out younger and younger as gay, lesbian, or bisexual (one blogger on Huffington Post has been sharing her adventures as the mother of a boy who at the age of 7 announced that he was gay), those kinds of assumptions just aren't appropriate. Gay people are gay regardless of whether they are or have ever had or ever intend to have same-sex sex.
Now, sometimes gay people have made unwise sexual choices and have to deal with the consequences, or are trying to decide if it's okay to start a same-sex sexual relationship. But, you know, if a woman gets pregnant outside of marriage and has to make difficult choices for what to do, we don't say she's "struggling with heterosexuality." A man who is facing a divorce because of an affair with a woman not his wife isn't "struggling with heterosexuality." A girl trying to decide if she should cave to pressure from her boyfriend to have sex (or vice-versa-- girls aren't innocent of putting pressure on boys for sex) isn't "struggling with heterosexuality." We call those things what they are. We don't wrap the blame up in the person's sexual identity.
The truth is, it's us straight people who struggle with homosexuality. We don't understand the attractions they feel. We're not sure where our religious convictions should lie. Sometimes it makes us uncomfortable. We don't know how to explain those two men kissing to our kids. Sometimes our assumptions of heterosexual orientation in another person get quite flattened and we are caught off-guard and feeling a bit foolish that we didn't know or hurt that we weren't told earlier. We struggle with homosexuality.
But gay people? Not so much.