Its something of a myth but not completely.
For women, if the hymen is still intact it can tear during the first time having penetrative sex and that can be painful plus accompanied by blood. However it is a myth that the hymen is intact in all virgins and that the first time is painful for all women.
For men, the first time penetrating someone can rarely be painful too if they have not retracted their foreskin much before or it is tight. Some men don’t realise they can retract their foreskin fully when masturbating for example, and may not be told by anyone (particularly is homes that may by uptight or even puritanical about sex education). So it does genuinely happen that people find out when they first have sex. For most men the foreskin will stretch a bit over time and tightness or pain will resolve but occasionally they need a circumcision.
Men with circumcisions may also experience pain the first time if theyve has a bad circumcision and the scar is restricting them or being pulled on during penetration. Again that usually settles but very rarely may need surgery.
Meanwhile for anal penetration, the first time is often painful largely due to inexperience. People are not used to relaxing the anus, or are nervous and tighten it. Also if they have a bad partner they may be too aggressive which can be very painful, or they may not use enough lube.
So yes the first time can be painful, but is a myth that it must be painful or is always painful. Pain can be avoided . Persistent pain every time sex is had may be due to poor technique but may also warrant a medical check up - for either gender/sex.
As long as they’re both of an age to consent legally, then yes, absolutely. Not sure what jurisdiction you’re in but 16 is age of consent in most places, and they’re going to do it if they want to do it. It’s far better to treat them like adults, let them be somewhere safe, and teach them that they should not feel shameful or fearful about that part of them. If the age of consent where you are is 18, or Tiffany is not legally of age, then the situation would be quite different and difficult but would have to be no.
That your son is able to be so honest and comfortable with you, and Tiffany with her parents (hopefully!) is a credit to you all. This is an opportunity to encourage him to practice safe sex and also that sex as part of a healthy relationship like this is healthy and valuable in itself (as opposed to sneaking around, and him not able or even bothered to provide a safe space for his girlfriend). It’s worth encouraging this attitude that to do this they should be somewhere safe, and comfortable, such as his room in your home. It will also ensure a much better experience for both of them should it be a first time (whether overall or just together).
I would just consider confirming that Tiffany’s parents are aware of this and comfortable with this; especially as at 17 are in a grey area socially between being legally adults in many areas but not others. EDIT: As someone else suggests, ask Tiffany to get her parents to call you - so that the ball is in her court.
I would also even consider being out until late so they can have some privacy. I would also set some ground rules such as no drinking or drugs in exchange for this, and no one else in the house while you’re out. I totally accept that may not instinctively sit well with you to be out but they’re both at the cusp of adulthood and total autonomy (literally months away if your son is 17) and already have autonomy in this decision (unless you’re in one of the few places where the age of consent is 18).
This sort of treatment, respect and trust is something your son will remember for the rest of his life.