Adopted brothers izuku & hitoshi - fell in love at young age -> become boyfriends (yes, incest)
Parents Erasermic are concerned but ultimately accepting, mostly just impressing on the boys that their relationship stays secret/hidden bc other ppl and society at large probably wouldnt take their relationship very well (what with it being "taboo" and people not reacting well to others being Different)
The boys do so, tho ofc they eventually get caught here and there (Midnight, nedzu, todoroki shouto, kaminari denki, all over time)
Eventually the boys make a plan w todoroki where shouto will fake date izuku and be his beard to give the aizawa boys an alibi and cover. (Shouto is aroace and it was half his idea.) They all move in together when graduated & tododeku is known to the public while the shindeku is not (as desired)
Eventually (nearly 10 years after the b/g of the fake date ~16/17yo) shouto proposes marriage to izuku (bc it feels about time they do or ppl will start getting curious), and the idea of marrying sho when he cant marry hitoshi upsets izuku until he gets together w shouta-dad and they decide on having a small intimate shindeku wedding ceremony (no paperwork but yes vows) before the big public tododeku one.
(They will have wedding necklaces instead of rings - maybe chain & pearl/stone bead?)
Kaminari (who found out when hitoshi vented to him while very drunk lol) is toshi's best man and shouto is izuku's. Kaminari is fine w the incest thing bc 1) hes a cery chill guy 2) he loves his friends more than anything 3) they were always pretty close in school so he supposes it makes sense but 4) he hasnt fully processed the situation yet lol (he'll still be fine after but it hasnt entirely hit, hes just going w the flow)