Use Me As You Wish [Saigenos]

As an omega, Genos wishes to please in whatever way he is able to. Before actually being claimed by his mate, Saitama's rut was extremely rough for the older man. He would break any toy with his strength, or they would wear out by his inhuman stamina. It's the reason why his sensei doesn't want Genos to be with him during his rut, he is scared that he'll accidentally hurt Genos.

There is also the fact that Genos is asexual. There are times that he wishes to have sex, but otherwise, he is neutral to such things. However, the thought of his mate using a toy or something else but him during his rut or otherwise makes Genos bare his teeth.

It's not until Genos accidentally stumbles upon an ad for a fleshlight does he come up with a solution. He asks Dr. Kuseno for his opinion and the man, thankfully, agrees.

When Saitama's rut hits does he go forward with his plan. If his mate is worried about it being too much for Genos then Genos will remove that worry. He lowers the sensitivity of his body and his omegan parts or made of a much more durable material. In other words, he will be no more than a sentient fleshlight for his mate.