Does the bible give specific instruction on marital sex? Are there sexual activities that are sinful? Various positions, "toys", fetishes, role playing, etc?
I read your answer about masturbation by the man struggling with homosexual desires but I have a slightly different twist. Is masturbation in general wrong? Also, I'm just wondering, what if a person is married but unable to have intercourse with their spouse for whatever reason, for sake of argument say one partner is a quadriplegic or maybe one spouse travels a lot for their job. What if that spouse masturbates but doesn't think of anyone but their spouse?
The Bible does not give specifics on marital sex. Essentially it says that sex should be between two married individuals (Hebrews 13:4) who are a man and a woman (Romans 1:26-28). What a married man and wife do in the privacy of their homes appears to be their own business. The Bible is not intended as a sex manual, and doesn't get into such specifics, as much as some churches would like to think otherwise.
Likewise it says nothing about masturbation, itself. As my previous answer indicated, if it causes either partner to deny the marital rights of the other, then it is wrong. Otherwise, the Bible is silent.
Many churches and preachers have said masturbation was wrong based on Genesis 38:8-10. "And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also." (Masturbation is sometimes also called "onaninsm.") What angered the Lord in this passage, though, was not that Onan "spilled his seed" on the ground, but that he did not follow the law that the brother was responsible to father a child with his brother's widow if she had not had a child already. This was to ensure family lines continued. Onan's sin was disobeying this law, not spilling his seed outside the woman.