Jesus never said anything about slavery either. (I'm sure there are also other issues that Jesus did not address directly during the time of His bodily incarnation here on earth.) But that does not mean that He affirmed* slavery. That does not mean He believed that certain men may take slaves. That does not mean that He agreed with the mistreatment of slaves by masters in a culture where slavery was permitted. That does not mean that He believed that some races or classes of men are below other races or classes of men; because they were designed to be that way by God. All the above conclusions are false.
The fact is slavery is a social injustice according to modern understandings and even according to the Bible. Slavery is more than social injustice it is against the very design of human beings as recorded in the Bible itself. All humans whether they be from any people group of the world or from any economic background are made in the image of God (Genesis 1:26-27). I am sure Jesus came to affirm this idea because He said that He came not to destroy the Old Testament but to fulfil it (
Matthew 5:17-18). We can conclude that Jesus affirmed Genesis 1:26-27 in His acceptance of disciples from different backgrounds, in His conversation with the woman from Samaria (John 4) and even His commission to spread His Gospel to the whole world (Matthew 28:18-20). If He rejected the idea that humans were made in God's image He would have rejected some of His disciples, He would have agreed with the Samaritan woman when she brought out the issues between Jews and Samaritans and He would tell His disciples to preach the Gospel only to selected nations. We can see examples of this in other passages as well.
The same goes for homosexuality. Yes, Jesus never spoke about it. But that does not mean that we can conclude that Jesus affirmed homosexual lifestyle just because He was silent about it. Again, why would He go against the very Scriptures He came to fulfil? Genesis 1:26-28 also affirms that God designed men and women to have only one sexual orientation for the purpose of having progeny. If Jesus supported verses 26 on the issue of humans being made in the image of God then naturally we may conclude that He also agreed with verse 27 (Matthew 19:4).
Some "Christian" groups may say that this answer is homophobic or say that I am homophobic. But the fact is that the argument that if Jesus never spoke about (either against or for) homosexuality then Christians should also not say anything about it is a dishonest argument*. Also, another fact is that the argument that if Jesus never spoke about homosexuality then Christians should allow, if not affirm it, in their Christian communities and churches is also a dishonest argument. The truth is that Jesus came to fulfil the Old Testament not to reinterpret it according to human whims.
The argument that if Jesus never spoke about homosexuality then Christians should also not say anything about it or, at least, allow it in their communities and churches is true if you reject the authority of the Bible and the authenticity of Jesus of the Gospels. This happens in liberal theology and/or post-modern theology. So a lot of confusion will be avoided if the one who argues for such things from the Bible if he or she states where they stand on the authority of the Bible and the historicity of Jesus in the New Testament. Either they are confused or misguided Christians or simply lost who need Biblical guidance OR they are of the liberal or post-modern camp who still claim the label of "Christian". The second group also needs the Gospel!
If I may be allowed to argue the same way that the other person argues then let me state the following. I am sure Jesus was aware of the homosexual lifestyles and relationships of His time. But when He spoke about marriage and divorce (Matthew 5:31-32; Mark 10:11-12; Luke 16:18) He never mentioned the validity of homosexual lifestyle. He used the terms such as husbands and wives that excludes any idea of homosexual lifestyle. Therefore, Jesus never affirmed homosexual lifestyle. (This is a much better argument when we take into account what Jesus said in Matthew 5:17-18; this also affirms the traditional notion of marriage).
My argument from Matthew 5:17-18 may sound weak to some but I assure you no other statement is needed from Jesus than this to show that He came to fulfil the Old Testament not to destroy it - what liberal, post-modern and similar theologies seem to be doing.
*affirm = accept as truth and therefore allowed for practice
*dishonest argument = intellectual dishonesty that is a result of ignoring facts, truths and proofs, this happens because of laziness or because of misinformation or because of lack of information or because of intent