The real question isn't whether or not you get defensive, or who gets defensive. The real question is what you do with it. I will say these words often: S l o w E v e r y t h i n g D o w n. In the case of defensiveness, you can use the tool of slowing down so that you don't REACT defensively; instead you can wait and then RESPOND usefully.
Imagine if you will, a defensive reaction as a wave; it will rise, then it will fall. Your job is to breathe through the rise and fall, and if you have the where-with-all, to be transparent with your partner and say, "I'm feeling defensive, so I'm going to breathe and wait for the reaction to pass so I can respond well to you." (This doesn't usually happen early in couples counseling; it's a learned skill.) If you can't, just zip it until the wave falls.
How come you get defensive so quickly? Your limbic (emotional) brain responds in 1/200th of a second. As you slow down, you can notice what happens in your body; muscles tighten, heart rate increases, breathing changes. What happens in your body when you're feeling defensive?
Over time, you might begin to notice your bodily reactions earlier and check in with yourself: I'm noticing a physical change, what's happening? This may allow the wave to rise and fall more quickly. It may allow you to use your cortical (thinking) brain sooner, to be transparent and say that you're starting to feel triggered and need to slow down or take a break.
You already know that having a conversation when one or both of you is feeling defensive isn't a conversation; you can't express yourselves or listen well. If you need a break, take one -- please be sure you come back to talk with your partner so you can understand both what triggered you and whatever the original discussion was about.