Here is a recent article by Greg Koukl looking at whether the claim that "homosexuality is natural" actually stands up under scrutiny. I have only included the start of the article but it is well worth going to Stand To Reason and reading the full article because it is very likely that you will be faced with arguments like this.

Homosexuality Is Unnatural: An Is-Ought Fallacy?
Greg Koukl, Stand To Reason

"Recently a caller to the radio told me about a conversation he’d had about homosexuality.  The caller made the teleological argument, that looking at what the natural functions of the male and female reproductive organs are for, we can draw certain conclusions about how they should properly be used. The person he was talking with challenged his argument that you can’t get an “ought” from an “is”.  The challenger seemed to be saying that just because it is that way in nature doesn’t mean that we can derive a moral rule from it.  The caller asked if the challenge was incorrect and how to respond to it.

On the principle the challenger is correct in describing the is-ought fallacy.  But rather than working against the teleological argument, that principle works against a common argument in favor of homosexuality, which is, if homosexual interests are natural to someone, they are therefore morally acceptable. That is an example of an is-ought fallacy.

The is-ought fallacy, first articulated, by David Hume is put simply as you can’t get an ‘ought’ from an ‘is.’ The more precise way of characterizing it is this; You cannot have a syllogism that has a moral term in the conclusion if there is no moral term in the premises. To be a valid argument, the conclusion has to follow from the premises. You can’t have anything in the conclusion that isn’t already set up in the premises. Hume identified this particular fallacy in arguments that were based on mere descriptive elements but had a conclusion with moral terms in it.  That is the is-ought fallacy.

People sometimes argue in favor of homosexuality by arguing that their inclination is natural, and if it’s natural, then we shouldn’t be making any moral objections about it. If that is their argument they are guilty of is-ought...."

Click here to go to the full article on Stand To Reason.