I once heard someone speculate that some of the most outspoken homophobes who think sexual orientation is a choice are actually gay or bisexual. So they are people who have resisted their homosexual urges to identify as heterosexual. (How happy they are playing straight or whether they slip up or not is a different question.)
Therefore, they think everyone has had those urges and makes a deliberate decision to be gay or straight. The ones who choose to be straight (like them) have made the right decision, while those who are gay have made that choice as a result of being weak, hedonistic, sinful, mentally ill, or all of the above.