They say the same thing to those with other sinful desires. Why is it loving to say "I know you've desired to live without eyes your whole life, but you need to accept that God didn't give you that cross" but unloving to say "I know you desire to act sexually with a member of the same sex, but God didn't make sexual relations for the purpose of unity only"?
Now explain why a sterile man and woman or a pair of hetero 65 year olds marrying isn’t “against nature” or a thwarting of the primary “reproductive purpose” of marriage.
Biblically, sex should only take place within a marriage. If it takes place within a marriage, the sex is sanctified, and non-sinful. Any sex that takes place outside the context of a marriage, is inherently sinful. That means adultery, abuse, and so on. Same-sex individuals simply cannot marry within the Biblical meaning of the term, which means that their sexual activity will not be sanctified, and will therefore be a sinful action by nature of what it is.
Being a homosexual or feeling attraction to your own sex is not sinful - it's a very difficult temptation that, with God's help, you are supposed not to give into. It's acting on this same-sex attraction that is sinful, in much the same way as acting on attraction to your neigbor's wife would be sinful.
