I can only speculate at best, that a gratifying motive would be hypothetically sinful I suppose, but that is dependent on my own ideology only;
Such an ideology would not be based on the Bible as far as I know. I can think of no scripture, other than the OT story of God striking down someone for "pulling out", that describes non-procreative sex within marriage as fornication. Then again there was not really any such thing as non-procreative sex at the time except for some crude IUD and abortion methods.
What the Bible DOES say though is that if you are married you should get jiggy with it often! I can't find the verse at the moment but there is somewhere it even says that you shouldn't deny your spouse for more than 2 weeks. It does so, and many argue, for the purpose of describing a healthy marriage relationship. Certainly, since a woman is only fertile for a small window of opportunity every month, a lot of the sex the Bible says you should do would be non-procreative.
Really then, the argument that same-sex marriage is sinful because the sex acts are not "covered" by the exception of procreation can be argued has no real Biblical basis.
That being said, there are OTHER reason in the NT that can be used in particular to argue that same-sex marrige is a sin.
There is also plenty of justification in the NT to suggest that other things such as looking lustfully at someone is the equivalent to adultry. But you don't hear most (and I must clarify most because I know there are plenty of scary Christians that do) Christians saying we should repeal free speech so that we can censor all media/books/speech that may cause someone to think "bad" thoughts. Most Christians I feel would agree that THAT sin is not something we as men should codify into law because God is the one that judges the sins of the heart.
There is no difference between that and same-sex marriage. If homosexuality really is a sin in the eyes of God then it is a sin of the heart, not against fellow man like theft, lying, and murder. It is therefore not up to us to judge homosexuals or restrict them from the other worldy benefits that heterosexual people enjoy. A free society does not go against God, people go against God. There is no reason Biblicaly or otherwise to disallow legal unions between same-sex couples.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)