From what I'm reading that's the problem. If you go above friendship status to good friend or best friend then you can't become romantically involved with them.
NEVER get to the best friend stage or the good friend! if you already have gotten there be rude and make them your enemys. then you need to start from the beggining and make them friends then flirt, tell flirty joke, pick up line. When you are at the partener stage you should be able to woohoo if you go on be romantic then scroll down.
Source: http://www.chaptercheats.com/qna/iphone-ipod/29463/The-Sims-3-Answers.htm?qid=9203
Hope that helps! 🙂