slmz,
i would just like to say, that fair enough, he is not married yet, but the fact is that he is engaged! thus he is no longer single........
being engaged, and being single are two different things.
also, if he has to make mention to this girl when he saw her, in passing by at work, something about her bum, that couldve been taken as a joke. i understand your concerns, as he went out of his way to sms her and wrote that comment about her bum.
there has to be boundaries to the way we communicate wit others, especially if we have a fiance, wife, husband etc. because of respect for them, we need to take caution as to the way we reply to messages, or talk to people off the opposite sex. if we ourselves do not have respect for the person we are going to marry, how can others be expected to show them respect!
harmless flirting can lead to someone close to us being hurt. so for that reason, its best to look out for the feelings of our loved ones, and for that reason, i feel what your fiance did is inappropriate. and he IS NOT SINGLE! he is your fiance! (to the people who commented saying he is single).
speak to him about it, he may see your point of view and understand where u are coming from and refrain from this type of behaviour going forward. i am sure he would not like you messaging guys at your work about their 6-pack etc.