Do Christian theologians believe that God will punish secular humanist for their sexual behaviours, including premarital sex and same-sex relationships and forgive them of their sins because they have different beliefs?
There are several distinct issues being conflated in this question. The first issue is whether Christian theologians believe that God punishes people for certain forms of sexual behavior, and if so what forms. This is a subject of some contention among Christians, with some denominations performing blessings of same sex unions and some strongly opposed to them. Thus the only accurate answer is that Christians differ in their beliefs on this subject.
Next, there is a question of whether homosexual and extra-marital sex are solely performed by secular humanists or whether they exist in the Christian population as well. As the recent scandals in the Roman Catholic Church have shown, people of all religious persuasions are capable of rape, pedophilia, homosexuality and a variety of other behaviors; in fact, extra-marital sex is actually just as common among non-Christians as Christians. According to most Christian theologians, this would be explained by our all being sinners.
Finally, the most interesting part of the question is whether God would consider a hypocrite, who performed homosexual acts while preaching an anti-gay gospel a worse sinner than a gay atheist. Given what Jesus had to say about hypocrites, many Christian theologians might argue that claiming to be a Christian but committing adultery, fornication, or other acts you have promised God you will not commit, is doubly problematic.