Is adultery a sin which some are much inclined to?
Yes.

They were as fed horses, every one neighing after his neighbour’s wife,
Jeremiah 5:8

Is it necessary therefore that it should be forbidden?
Yes.

The law was made for whoremongers,
1 Timothy 1:10

Was it forbidden under the Old Testament?
Yes.

Thou shalt not lie with thy neighbour’s wife to defile thyself with her,
Leviticus 18:20

And is it forbidden under the New?
Yes.

For this is the will of God, that ye should abstain from fornication,
1 Thessalonians 4:3

And is it contrary to the law of nature?
Yes.

For Abimelech calls adultery a great sin,
Genesis 20:9