Uhhh, it's like I just looked up an encyclopedia entry/decision chartif someone is in the younger age group, they should get the vaccine (i.e. they're 20 and have been in that relationship since 12) because they're statistically less likely to have been already exposed (both because of age and limited sexual history), and because there are no guarantees about what the future holds (breaking up/divorcing, cheating, or even a partner dying)
if they are beyond the key age group and they intend to become sexually active with someone(s) outside of that relationship, because they are less likely to have been exposed to HPV already there is a greater potential benefit to vaccination, so their doctor may still recommend it, but that potential benefit is more limited if the relationship it's continued and monogamy is maintained.
both vaccines and the things they protect against come with a variety of potential complications or side effects, so ultimately whether a vaccine is necessary or recommended or not comes down to a variety of factors like how bad would it be if I got full blown [illness] and what is my potential risk for exposure to [thing being vaccinated against]. some things, like Polio, are so terrible that even with a lower likelihood of exposure, the (minimal) risks of the vaccine are nothing compared to the risks of contracting polio. for something like HPV, the risks are less uniform, so it relies on now shared clinical decision-making that will vary from patient to patient depending on their unique clinical factors and risks.
we handle some other vaccines similarly, where the vaccines exist and are approved for particular age ranges, but are not typically administered for parts of those ranges unless particular aggravating factors are present. like, adults 65+ are supposed to receive at least one pneumococcal vaccine. however, if certain medical conditions or other factors are present, it may also be recommended to receive additional doses, and earlier. there's a medium risk tier, where it's recommended to receive one or two doses between 19-64 and then the regular recommended dose at age 65 (some conditions that put you in this medium-risk category include asthma, smoking, alcoholism, chronic heart, lung, or liver disease, or diabetes mellitus). there's also a higher risk tier where you are recommended to have three or four additional doses before the regular 65+ dose (these conditions include CSF leaks, cochlear implants, chronic renal failure, and sickle cell/hemoglobinopathies, as well as congenital or acquired asplenia or immunodeficiencies). so it's *technically* safe to have as many as 5 pneumococcal vaccines from the age of 19 on, but we don't administer that many of those vaccines to EVERYBODY because the risks of exposure and severe illness for the average person under 65 are less than the risks of vaccination. for those with the listed conditions, the vaccine is less risky than rawdogging the bacteria is.