There is nothing that says thinking about sex is wrong - Fornication & Adultery - and looking lustfully. Man and women were created as partners & to multiply. - the OT & Jesus were pro marriage - Matthew 19:4-5 - 4. "Haven't you read," he (Jesus) replied, "that at the beginning the Creator `made them male and female,' 5. and said, `For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh' ?
Paul - mentions better not to marry - unless you cannot control yourself - so basically for sex get married - no love / family etc... just sex.
1 Corinthians 7:2: 2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.