Is it haram to think about sexual things without acting upon them? What about homosexual thoughts?
The Prophet, , says:
God has pardoned for my community what comes to their mind, so long as they do not act or pronounce words to that effect. (Sunan Abi Dawud 2209, various forms of this hadith are also present al-Bukhari, Muslim and others)
However, as Muslims we should always aim high and try to do what is most admirable. Your goal should be to try to understand God and to work to become the type of person that is most pleasing to Him. From this perspective you can then judge for yourself what thoughts are good and what thoughts are not.