Often the homosexual marries the heterosexual in an attempt to "cure" their sexual attraction to the same sex. Religion, family expectations, and a desire to have children often influence a homosexual person's decision to wed a person of the opposite sex.
I use the term "homosexual" and not LGB because many same-sex attracted people that marry a person of the opposite sex do not self-identify as gay, lesbian, or bisexual. Many also prefer to stay in the closet.
Many, but not all, of these marriages end in severe marital conflict, separation, or divorce.