The presumption isn’t to asser for the sake of assering. Do you think the oilam doesn’t go to mixed gyms for no reason?
You can’t take something you know full well is considered assur by all mainstream Orthodox Jews and presume it’s muttar until you are shown sources to your satisfaction, for which there are no objective criteria. This is clearly and obviously something which falls into the category of a huge nisayon which is assur to put yourself into, and to take something with obvious reason to asser, which is clearly assumed by the frum velt to be assur, and call it presumably muttar, is blatantly wrong.
Comparing it to standing on your head, which nobody considers assur and there’s no reason to think is assur, is ridiculous.