@awisse17,
Yes an external factor is sex. I'm not clear what you think the answer is, stop people having sex? Or Africans? Or just gays? Semen is the big factor in sexually transmitted HIV, making lesbian sex much safer for women in this respect.
There are a number of external reasons for a higher proportion of gay men catching STIs. Both people able to give and take semen is one reason. A far, far smaller group having sex with each other than straight people is another mathematical reason. Socially, I already said many are forced underground for brief secret encounters because others disapprove.
Gay people are also not held to 'hetero' norms of marriage and kids. How many marriages stay together for the kids? There's an increase in older divorces because the kids leave home and Mum and Dad finally admit after 20 years that they can't stand each other. For unsuccessful gay couples they have no kids, no marriage, no reason to stay if it goes wrong. There is less expectation of life long commitment. However, where open gay couples are socially accepted, officially recognized and marriage allowed, monogamous gay relationships are far more prevalent and successful.
These are external social issues, and not inherent to the hearts, minds, or bodies of homosexuals.