Is this a trick question? 
Not at all. I say it is. If I'm not homosexual and I engage in homosexual relations, apparently that's a sin according to the comment above. So if I'm heterosexual and engage in sex outside of marriage (pre-marital or extra-marital), is that a sin? Honest question I believe.
My pastoral answer is that sex outside a covenant of love and faithfulness is not the fullness of God's gracious will for our gift of sexuality (gay or straight). Such a covenant is the heart of what a marriage is. If the covenant exists but is not legal I would be slow to call it sin. On the other hand, if the legal status exists without the covenant, then I would lean in the direction of thinking it sinful.
SPS
By what authority do you discard former (and for many Christians, current) understanding of what "the heart of marriage" is, and what God's gracious will is for our human sexuality? As I have said here before, I believe you accurately capture the unitive purpose, that is of expressing love and in uniting the two, but I believe you reject the procreative aspect of the marital act. It is the rejection of one of the fundamental purposes of sex within marriage where I believe you are in error.
For an explanation far better than I can explain, see the USCCB 2009 document on Marriage:
http://www.nccbuscc.org/loveandlife/MarriageFINAL.pdfPage 11: The Two Ends or Purposes of Marriage
"Marriage has two fundamental ends or purposes towards which it is oriented, namely, the good of the spouses as well as the procreation of children. Thus, the Church teaches that marriage is both unitive and procreative, and that it is inseparably both."
Any sexual act can be understood in this context. Is it unitive (selfish or giving), and is it procreative (open to and capable of creating life).
Anything which denies either of these, denies God's full intention.
masturbation (not unitive or open to life)?
contraception (not open to life)?
adultry (not unitive)?
pornography (not open to life nor unitive)?
homosexual acts (not open to life)
... add your favorite here..
I believe until fairly modern times (last century) there has been universal understanding of this by all Christians. So either the Holy Spirit is opening our minds to a new understanding, or some other spirit is once again at work to deceive us from the fullness that God truly intends for us.