If God is all mighty,why doesn't he change bad people/seiners into good ones?

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Church forbids sex for pleasure even with a spouse.

1. Which church, and what's your support?

2. Red herring. If people followed the church's rules, AIDS would not be a problem. The argument that the Catholic opposition to birth control fuels the spread of AIDS is possibly the stupidest atheist argument going.

1. The Catholic Church. The granddaddy to whatever denomination you follow, unless of course you are a Coptic or Orthodox.

2. What about the indigenous belief systems of the peoples of Africa? What about their cultures? They don't count? They've been around for thousands of years, and these people should just scrap them because they are invalid in the face of Christianity? I take serious issue with people who go to other cultures and basically tell them, "Yep, everything you believe is wrong. You are wrong. Your ancestors were wrong, and since they weren't Christians they're in hell or purgatory now, which is where you'll go too if you don't accept Christ as your Lord and Savior!"
Catholicism is on a rapid rise throughout parts of Africa. The use of condoms significantly lowers rate at which HIV is spread. Telling people it's sinful to use condoms increases the rate of infection. Because, surprise!! People tend to adopt certain aspects of their new faith, but often still follow their traditional ways as well. Meaning, they'll go to Mass and confession, but not necessarily give up their mistresses, which has a long tradition in many African cultures.
We can argue about the validity of African cultures or how much "better" Western culture is, but the fact is people like to hold on to their cultures as it is a part of their larger sense of identity. There isn't a continuum of "good" to "bad" cultures, it's just cultural differences.