Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge. ~Hebrews 13:4
Some time ago a Christian acquaintance contended that Christians “defile” the marriage bed if they engage in certain types of sex and he then proceeded to offer what is “clean vs. unclean” relations between husband and wife.
Yet the author of Hebrews offers no qualifications. He merely affirms that marital sex is "undefiled."
Two things come to mind which make sex marital:
1) It must be exclusive (between husband and wife only...no others [“no others” includes pornography]); and 2) It must be consensual (neither the husband nor the wife should be forced or manipulated [physically or emotionally, etc.] to do that which the husband/wife deems degrading or hurtful).
Within the above biblical boundaries, this boils down to individual preference and/or conscience. And in this matter—as in all matters—we must be cautious to not elevate our scruples above those of scripture. That is, certain parameters may be fine for one’s own marriage but should not be used as a universal standard.
Our authoritative, universal standard for Christian conduct is the Bible.
However, I am unaware of any passages in scripture which give guidelines for what is acceptable sex between husband and wife (beyond Moses' proscription of coitus during menstruation, which of course has to do with laws of separation and sanctification, not morality as such).
Hence, the notion of "clean vs. unclean" sex within the context of Christian marriage is uncertain and invalid. In fact, in the Old Testament, intercourse of any kind (which involved male ejaculation) rendered the husband and wife "unclean."
"When a woman lies with a man, and there is an emission of semen, they shall bathe with water, and be unclean until evening" (Lev 15:18).
When Moses commanded purification he said, "Be ready for the third day; do not come near your wives" (Ex. 19:15).
Thus, "clean" and "unclean" have to do with laws of separation and/or sanctification. These we believe to be fulfilled or completed in Christ.
And so, as long as husband and wife are within the proffered parameters of marital sex (see 1 and 2 above), I know of no scriptural reason, and therefore feel no compulsion whatsoever, to intrude upon them.
Let the husband render to his wife the affection due her,
and likewise also the wife to her husband. ~1Corinthians 7:3