simple "regular" dforce undies
There are many undies (female or male) which are way too much sophisticated.
Simple dforce bras with no frills but a realsitic back clip openable would be great ! Same for panties simple no frills dforce, with some "unhiding" morphs without deformations in the nether hip area when grafts are used for G9 or G8.1 f
And why not for G8.1 males and G9 counterparts : either the old kangaroo style, or plain boxers usable with men grafts gens insinde ?
That's, of course, for educational use....

Comments
For men, I would recommend for G9 https://www.daz3d.com/dforce-comfy-men-underwear-for-genesis-9 and for G8/8.1 https://www.daz3d.com/intimates-for-genesis-8-male-s
Editing to add: I don't know if the g9 ones can be dforced, but I have had good luck with dforcing the G8 shirts in the above pack, I haven't tried the underwear though. They do have plenty of bulge morphs to get them over the male parts, if desired.