Housewife and the NTR Onsen Vacation Anthology, the (18+ doujin)

Yumiko is happily married to her husband Hiro, and the two of them are going on vacation soon: their whole neighborhood is going to rent out a traditional Japanese inn, complete with natural hot springs! She volunteers to inspect the inn beforehand with the elderly neighborhood association president and his wife, but it ends up just Yumiko and the president after his wife catches a cold. On their first night together, Yumiko has a bit too much to drink… and the president is more than happy to take advantage of their time together! As much as Yumiko knows she shouldn’t, she finds herself enjoying the president’s size and stamina—and unable to resist, her onsen inspection trip turns into one of onsen infidelity! The inspection eventually ends, and Yumiko is eager to return with her husband so they can enjoy some moments of marital intimacy… but the president isn’t so eager to let her forget, and it may be harder than Yumiko thinks for her to stay away…