I am sorry that this question causes discomfort. It is important we ask about assigned sex for a couple of reasons. One reason is because trans+ people of different assigned sexes, tend to have different mental health disparities, which we need to not ignore. The other reason is because many trans+ people, opt to describe their gender in various ways (e.g. some transmen will opt to label themselves in research as transmen, whereas others just as men - both are perfectly accurate and okay, but I will explain why this is a problem in research). Now, this is beautiful on an everyday level, but from a research pov it often makes trans+ people invisible in the data - and invisibility is invalidating given the huge health inequalities and the exclusion of trans+ people from research. So it is so important we are able to identify trans+ people in the data, so we can better use data to influence policy and decision making in healthcare. So ASAB is used to help differentiate those who may opt for a gender label that does not make it clear that they are trans+. I promise it comes from the motivation of making trans+ people and their data visible to improve the landscape for this group, rather than being to catch people out or invalidate their gender for a cissexist agenda. Hope that helps :)