Satan had thrown a wrench into God's plan to have a perfect creation. Where God had created sex for the purpose of procreation, Satan had perverted it by causing man to serve creation instead of the Creator. Henry went on to say that, since the eyes are the windows of the soul, to lustfully look upon a woman would be feeding the heart the forbidden fruit. This means if the desire is not resisted and extinguished by the Holy Spirit, it will lead to sin and then to spiritual death. Sexual temptations are hard to resist because they play on the normal and natural desires the Lord has given us.
Sex becomes the forbidden fruit when it is practiced outside a marital relationship.
Touching and fondling a woman in a sexual way isn't permitted either because it will usually lead to committing the act. When we commit such an act, someone always gets hurt. But, more important, it breaks God's heart because it shows Him that we would rather follow our own desires instead of the leading of the Holy Spirit. The reason is that we don't own our bodies. Christ bought and paid for them with His own blood on the cross.