The Alliance Church’s official doctrine appears to be something like this.
Healthy sexuality is essentially procreative. Recreational sexuality is not procreative and not healthy. Sexuality that is not healthy is sinful.
Sexuality that is not fundamentally procreative is sinful. Homosexuality is not procreative. Homosexuality is therefore a sin.
Pornography is also sinful. Humans that deliberately become aroused by viewing or listening or reading pornography are also committing a sin.
Promiscuity is sinful.
Foundation in scripture
The foundation of this reasoning appears to come from scripture in the old and new testament.1
- http://www.cmalliance.org/about/beliefs/perspectives/human-sexuality ↩