"I don't agree with the gay lifestyle" suggests that you believe gay people are making a conscious choice to be attracted to others of the same sex. It implies that you are accusing us of being attention-seekers. Human sexuality is central (albeit not exclusive) to most people's existence. That applies to gay people the same way it applies to straight people.

If you don't understand homosexuality or can't relate to the experience of being gay, then simply say, "I don't understand homosexuality."