Question:

If just thinking about doing oral sex with a woman is as bad as doing it, then why not just do it?

Answer:

To illustrate why your argument fails, let me make a similar one: "If hatred is just as bad as murder (I John 3:15), then why not just commit murder?" The answer to this question and yours is that it isn't done because it is wrong. "Do you not know that to whom you present yourselves slaves to obey, you are that one's slaves whom you obey, whether of sin leading to death, or of obedience leading to righteousness?" (Romans 6:16). All sin leads to death and so should be avoided.

Jesus point in "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28), is that sin doesn't occur just in the physical action. Sin is present when a person begins planning to commit sin in his mind. "What comes out of a man, that defiles a man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:20-23).

Therefore, if think about having oral sex with a woman you are not married to is a sin, then stop sending your thoughts into the sewer.