This is red herring.
Most States and most countries have Age of Consent laws that except sex between like age minors (the age disparities range is usually 3 years). The lowest age of consent for minor-minor sex is, in most states, 12 or 13.
These laws DO NOT apply to adults having consensual sex with minors (except as noted above; 18 year olds with minors as young as 15, 19 year olds with minors as young as 16, 20 year olds with minors as old as 17).
Laws that aim to lower the age of consent without like-age provisions are NOT intended to protect children. They are intended for just the opposite.
You are exactly right.
In TX the age of consent is 17.
However, if the two copulating people are within three years of age then the older person cannot be prosecuted based on the sole fact that the younger one was under 17.
The tough cases (as a defense attorney) are when they are 18 and 15 and their ages are three years and a few days apart.
But yes the arguments for lowering the age are red herrings. Significant evil is at work.