'MTB couple' is a term used to describe two people who are likely to end up in a relationship to the point that they seem 'destined' to be together.
It is typically used in referral to characters in romantic films/ongiong television series. In these instances, it is blantant to the audience that the two characters will eventually get together, often in spite of numerous (frequently unnecessary) complications.
The two people in an MTB couple are seen as 'supposed' to be together (i.e. soulmates) and a relationship between the two is virtually inevitable.
Further complications generally arise when the two do get together, with the 'MTB' aspect being that they will always stay with/return to 'the one'.
Rom-coms always feature MTB couples who are initially separate for completely POINTLESS reasons - A woman is getting married to a man she doesn't even like and then suddenly realises she's in love with *Mark*, who has been in love with her since they were 5 years old, when she rejected him because she didn't like his shoes.
With the increased popularity of padded bicycling shorts, the "mount taint" phenomenon has decreased in frequency (though by no means disappeared), and many cyclists nowadays are unaware of the origin of the term MTB, believing instead in a folk etymology that describes it as a nonsensical acronym of *"mountain bike" ("MounTain Bike"?).
Bob: No kidding! I had to see my proctologist last week... now I get why they are called MTBs!