This is a difficult topic to address without using graphic language, but you basically stated what I believe.
When it comes to what is and is not permissible in "the marriage bed," the Bible does not always explicitly state the same. Instead, sometimes (often), we need to use deductive reasoning.
For example, when it comes to the topic of marriage, God said:
Mal 2:15
And did not he make one? Yet had he the residue of the spirit. And wherefore one? That he might seek a godly seed. Therefore take heed to your spirit, and let none deal treacherously against the wife of his youth.
One reason why God ordained marriage, or one reason why God made the two one, is because he desires godly seed or godly offspring. Of course, in order for godly seed, as in godly children, to exist, another type of "seed," or a man's sperm, must be involved in the procreation process. I will try to say this as delicately as possible, but, as we all know, throughout the world, a heterosexual, married man's "seed" will normally wind up inside one of three orifices in his wife. In only one of those orifices can his "seed" truly take root, or in only one of those three orifices can it potentially contribute to the procreation process. In the other two orifices, it never can.
Using deductive reasoning, this makes me believe that this one orifice is the orifice that God designed to be in contact with the man's "seed," and it also causes me to seriously consider that the use of the other two orifices are perversions of God's original intent.
Whew!
Can I at least get a "like" for my semi-non-descriptive use of terms?
Talk about walking on eggshells!!!