Homosexual practice is contrary to God's purpose for creation. He created human beings "male and female" in his image. Thus, even if Scripture were silent on the issue (which it is not), the abhorrence of this practice would be obvious from the witness of creation itself. This is an objective truth, decreed from the foundation of the world.
"Gay" activists, however, try to skirt the issue by framing it in subjective terms. "Can one be a Christian and gay at the same time?" is a misleading question. Many Christians struggle with this or that particular sin, even after justification. Old habits are hard to break, even after a life-transforming experience. Our old self still wars with our new self, seeking again to find a foothold. Those truly committed to seeking perfection, however, will rely fully on Christ, through the power of the Holy Spirit, to deliver them out of their former state, fully and completely, to stand at last sanctified and made whole. Inevitably, there will be some stumbling along the road to wholeness, but the truly repentant heart will continue to trust in Christ and reach forward toward perfection. Those who seek to find a way to continue being unrepentantly "gay" after becoming Christian, however, are not fully relying on Christ to deliver them from their sin and, thus, are not truly seeking after the holiness without which no one will see God.