You cannot categorically say that “Christianity” believes that homosexuality is a sin and homosexual marriage is not accepted when two mainstream Christian sects, the Presbyterians and Episcopals, accept it. All you can say is that your Christian sect and God as your Christian sect understands him.
You cannot say that homosexuality is not natural when studies have pinpointed patterns in two regions of the human genome - one on the X chromosome and one on chromosome 8 as having a tie to male homosexuality. (Also certain hormone levels in mothers while they are pregnant have been found to have a tie with homosexuality.)
http://www.natureworldnews.com/articles/10443/20141118/homosexuality-genetic-strongest-evidence.htm
Marriage is not simply a spiritual union. It is a legal and economic one. It carries with it spousal rights to inheritance, tax benefits, hospital visitation and health decisions, property rights, child visitation rights (and child support) etc. This is why a civil union or marriage should be allowed. The new Irish law allows any two adults to enter into such a formal union and does not affect the religious marriages allowed by any religion.
The argument against gay adoption is likely fear based. I think a very large number of children from heterosexual relationships are now raised by one parent or outside of a marriage relationship.
Predisposition does not equal behavior. So tell me, I am genetically autistic (around 100 percent correlation in identical twins, where they are both autistic), so does that justify me not trying to control my behavior despite being autistic? Does genetics justify homosexual men having bathhouse orgies, engaging in super-promiscuous behavior with 100s of partners as is the case for quite a number of homosexual men per the CDC, or should they take measures to actually master and control their actual behavior regardless? Where is the line? Because that answer has never been given to me about what genetics justifies in terms of behavior, NEVER EVER HAS.