I also do not take offense often, but this comes with privilege. I can handle a little joking around about MY situation, and I am more than willing to joke right back.
The problem arises from generalized statements about the transgender community. This casual transphobia is what gets us KILLED. This speech socializes everyone to believe that transgender = bad. It is the reason suicide can seem like a more realistic option than transition. It is the reason people are hesitant to date us, no matter how open minded they believe themselves to be. It is the reason we are targets for harassment and violence every time we walk out the door.
The ability to change this way of thinking starts with AWARENESS of the things you say. Many are not aware of how our speech can affect others. By pointing out signs of transphobia to others, you help them become aware and change their own patterns of speech. This helps them to become better allies, and one day they will be able to correct others as well. Imagine the day when you are with a group of friends and one says "just make sure your date doesn't have a dick" to try to get some laughter, and another friend pipes in "saying things like that are what get trans people killed". That is an excellent ally who shut down the laughter and brought awareness to the group.
Always check your privilege. You may have the ability to be around accepting friends and joke about this stuff in a safe environment, whereas someone is out there on the streets, rejected by friends and family, struggling to survive. I would challenge you to use this privilege as a way to spread change for the better of all of us, and it starts by pointing out transphobic speech.