Just to pick up on your point about two consenting adults. Let’s say the law of the land changes and now the legal age of consent changes from 18 to 14, would you oppose a 40 year old man sleeping with his 14 year old daughter? After all, it’s two consenting legal adults.
Your views on morality will always remain inconsistent because it relies on the law of the land. For example, you may be in support of the law where anyone under 18 cannot drink alcohol, but if that law changed then legally you would have to be ok with your child drinking at the age of 15. And, if you go against it then you’re not very “open”, are you?
OK I will bite ...
I have heard such logic before, from some of my own extended family members. The core of this argument has consistent flaws and let me try to explain. All I'm requesting from you is that you don't see this as a non-muslim or a non-religious person trying to score a point against you because that is really not the case, and for you to see my point with an open mind. Fair enough? I will extend you the same courtesy of course.
Let's say there are two solutions - religious solution (yours) and humanitarian solution (mine).
Flaw #1 - Perfection Flaw
Whenever we are presented with an opposing theories, especially when it contradicts an entrenched or an emotive opinion, we seek perfection in the other choice to be incentive enough for us to switch over (emotional switching cost?) knowing full well at our subconscious level that no choice is perfect including our own. We see this prominently in election type scenarios where in my own country most people knowingly voted for a clown in 2016 even though they were not fully vested because the candidate of "the other party" was not perfect enough for them to switch. I see opinions on things like gay rights along the same lines when those opinions are formed as a result of emotive religious beliefs. You consciously or subconsciously know that no option is perfect, including your own religious solution (banning nominal rights for all law abiding normal gay people that form the fat part of the statistical bell curve) yet you seek perfection in order to be convinced by the "other choice". We both know that every choice has pros and cons. Thus the ideal scenario would be weight pros and cons and choosing the least expensive option (not waiting for the perfect option). Granted you could claim the same for me, right? But my contention is that I have no emotional stake in this at all (not religious, not gay), I do not seek to establish a certain lifestyle or belief system upon others and I'm looking at only as the best objective choice. If you claim your religious solution is better from an objective and data based standpoint, I'm open to listening and even changing over. Can you say the same for most religious based solution suggesters?
Flaw #2 - Extremist Flaw
This is possibly an offshoot as the emotional high switching cost for adopting something other than your own emotive belief. In order to create better comfort level with your own choice (all human brains love echo chambers), your mind is comparing an extreme corner case of the "other choice" with best case scenarios of your own choice. Hence your question about 40 year olds with 15 year olds. Your mind implicitly or explicitly is comparing best case scenarios of religious solution with lower probability worst case scenarios of humanitarian solution. Our societies have evolved to a great degree of human and personal freedom with significant studies in human mental development. I would say the age of 18 is set for almost all adult decisions in most places because it is an evidence based data point for the minimum time for a human to be held accountable for his/her decision. Bear in mind that human mind is not a binary switch and we do not flip from kid to adult. It is a gradual process and there are studies showing human brain continues to develop until 30. But as we know, treating everyone under 30 as a kid is not realistic given the current lifespans and social constructs. Thus 18 years gives a high enough percentage of development especially because this is not linear by age but exponential. Given these studies, the legal age being changed to a number lower than 18 is near impossible in almost all countries with normal civil laws (not considering places like Afghanistan or Somalia).
Directly to your question - Lets say IF that happens - if legal age is lowered and 40 year olds are predating on 15 year olds, then I will happily switch over to asking for banning it because this is backed by evidence based data. I will not blindly support it just because it is legal.
You implicitly said something important and I strongly agree with that - "
What is legal may not be what is right"
200 years ago legally women were burned with husbands, my own forefathers died from Spanish inquisitions. 100 years ago legally most of us were under some form of slavery. 50 years ago legally most of us cannot have access to proper education due to skin color. So, I do agree with your core point. But the positioning of your point and why you position it so seem to be flawed in this context.
Given the above, yes, I will stand with you with enough scientific and evidence based data. But can you say the same for your stance? With enough evidence based data from recent medical/social studies, will you be open to supporting gay rights? Or will your mind want to search for extreme future hypotheticals to seek the echo chamber comfort of current choice?
Sad truth is even when I present such an argument with some of my family folks they only resort to another level of extrapolating hypotheticals under the guise of convincing me with evidence based data when there is no factual stance for their argument.
Thanks to anyone who has read through this long post.