Noticed thinning on my temples and front hairline: MPB or supplement?

You're not doomed.  The reality is that the over-the-counter "testosterone boosters" are not going to increase your testosterone dramatically, but much more like very slightly.  Hair loss is not directly inherited from your mom's or dad's side of the family, but rather both, and it can skip generations.  So, you may be the lucky one.  If you're looking to build muscle and using supplements, then that's perfectly OK.  Propecia is an option for you to prevent any excess testosterone from being converted in your body to DHT, which is the hormone that causes hair loss in men with MPB.  If you are destined for MPB, taking testosterone or supplements designed to increase your testosterone levels will simply move you forward in terms of how soon your hair loss will occur.  Hair loss is a progressive phenomenon, the rate of which is affected by, say, increasing your testosterone levels.  LLLT (low level light laser therapy) can also be of great help in helping to slow the progression of hair loss.  Bottom line is that there is no cure for hair loss.  None.  So, now that you know you have it, it's important to remember that a hair restoration procedure is only a re-distribution of hair that happens to still be on your head; it doesn't create any new hair.  So, therefore, keeping the hair that's on your head right now is important.  There are several ways to manage the progression of hair loss, with Propecia and LLLT being the most effective, according to the published, controlled, scientific studies on the various therapies available to curb the progression of MPB.

Saying that you are "doomed" would be an over exaggeration. You need to be smart and understand the cause of hair loss.  If you have genetic balding, there isn't much you can do to make it stop or make it worse.  You can argue that testosterone supplements may have accelerated the natural genetic process, but it would have happened without it.

Without seeing your scalp, and knowing all your history, it's not possible to comment. When a given individual uses an over the counter boosting supplement, we also don't really know if clinically they truly had an increase in testosterone (measured by blood tests). Not all reliably do boost testosterone. But here are a few general comments for anyone who uses any supplement to increase testosterone.1. if you have a genetic predisposition, then yes, all sorts of testosterone based supplements and boosters (and anabolic steroids themselves) have a chance to precipitate genetic hair loss. The less you take and the shorter the duration the less overall effect. Increased shedding and thinning are the earliest signs. In some men the progress of hair loss continues even after the supplements are stopped. 2. Whether or not to start medical treatments to promote growth (such as minoxidil, finasteride, or laser or platelet rich plasma) needs to be discussed with a physician on a case by case basis.3. In the earliest stages, I often measure free and total testosterone as this can give a sense of where things are at baseline. 

You are not doomed.  You are just the right age to start getting a "mature hairline" AND to start having MPB.  You are also taking the right supplement to accelerate male pattern hair loss!  Either way, at age 25 it is unlikely you need any additional testosterone and it can be harmful even outside of its effects on your hair, so it is probably good to stop it if you care about your hair and your general health.  Incidentally, if this worries you that much - take Propecia!