I have known several people to take deer with a thermally sighted rifle. It is legal here in Texas, so long as you shoot during legal hunting hours.
HOWEVER, most of the deer hunters that I know that use thermal, just handheld spotters for the purpose of locating live deer before hunting in the morning (no need to hike through the middle of bedded deer in your pasture if you can walk around them on the way to the stand), or use them to locate downed deer, usually, but not always after dark.
Can you id the blob to BE SURE its a deer, not a pig, a cow, a dog, or a person?? Does that thermal sight let you see its a buck, and not a doe?? I never found a thermal sight that precise.
I take it that you haven't spent time with quality thermal recently. I have never mis-shot a hog thinking it was a deer, cow, dog, or person. The same rules apply when using thermal as apply when using daylight optics, night optics, or no optics. You still have to ID the target before you pull the trigger (if you want to be certain of your kill).
Whether or not you can ID buck from doe will depend on the environmentals. Antlers are not vascularized AFTER the velvet comes off and so while you can count points readily when they are in velvet, it can be difficult to do so when they lose the velvet (loss of heat supply), particularly if it is high humidity, misty, rainy, etc. However, that is mostly after dark. At times, the antler can become virtually invisible, much like the wires and t-post on a barbed wire fence in misty conditions. Of course, this is when you are approaching thermal gray-out where everything in the environment no generating heat is based homogenized to the same temperature by the moisture and lack of thermal radiance from sunshine.
In the day time during hunting hours, on a good, sunny day, with a decent thermal, you can count points certainly within typical hunting distances.
Now reynolds, you asked if it was ethical. Well, that is between you and your hunting partner. The law defines the rules and ethics you are legally obligated to follow (and yes, we put laws in place to maintain ethical standards and several states point this out in their descriptions of their game laws). Those are the only ethics you have to maintain. Beyond that, it is up to you. What you consider ethical versus what I consider ethical (in this case, I think it is fine), and what anyone else considers ethical may not overlap on this issue and that is fine, so long as you obey the law.
Ideally, people will scan for animals NOT using a rifle scope, but using a handheld. This isn't so much ethics or legality, but just safety.
Here is a little thermal handheld I have been testing, though this isn't my video. I have a couple of hunt videos out with it, but the focus is not on distinguishing species or features, but identification of hogs at distance (primarily). Anyway, this guy does a pretty good job showing what the little thermal can see. This is a small thermal, similar to a FLIR Scout like I had years ago, same resolution (384x288) which is not high end, better thermal sensitivity (able to detect more subtle changes in thermal), but has a tiny 19mm lens.
https://www.youtube.com/watch?v=Mk1vsjiK_r0
Here is where I use the same optic and discuss the issue of observation versus recognition versus identification...
https://www.youtube.com/watch?v=Bq39PaPO6hs