When I first started becoming more observant, I started fasting תענית בכורות/Fast of the Firstborn on erev Pesach. After I became fully observant, I learned that I had been wrong to fast on two counts:
- I am no longer a בכור so I didn't have to fast.
- The custom is for בכורים to attend a siyum in order to participate in a s'udas mitzvah and not to fast.
Live and learn.
The truth is that תענית בכורות is a strange fast anyway. It's the only fast where the minhag is to not fast. That is, attendance of a siyum is not seen as a "if you happen to attend a siyum you don't have to fast", instead it is the observance of choice. Groups plan ahead to have a siyum scheduled with the express purpose of getting the בכורים off the hook. I have seen two explanations of this phenomenon.
R' Shlomo Zalman Auerbach, ztz"l, explains that that the בכורים are fasting because originally it was the בכור in each family who was responsible for the avodah. They only lost that job to the Kohanim after the disaster of the חטא העגל. According to that, the fast has nothing to Pesach, so why is it observed on erev Pesach? On that day every family -- potentially millions of groups -- is bringing their lamb (or goat, I suppose) as far as they can, then turning the animal over to the Kohanim. The בכורים therefore feel their loss -- that loss of an opportunity to come closer to HaShem -- with a particular poignancy on that day. Their first response is to want to fast as repentance, which is certainly appropriate. However, they can do more: Their loss is an opportunity to come close to HaShem -- learning Torah also brings one closer to HaShem. Their attendance to a syum emphasizes that we can still come as close to HaShem as we want; the effort it up to us, but the channel is open. Their participation in a siyum, therefore, achieves the same goal as fasting.
The Chasam Sofer (thank you, again, Dirshu) says that the custom goes back to that very first Pesach in Mitrayim -- the Jewish בכורים were nervous about whether they had enough merit to make it through the night, so they fasted to do t'shuva before the fact. (Not much different than the תענית אסתר, actually.) This also explains why the father of a בכור fasts as well; he wants to save his son! The Chasam Sofer doesn't explain the custom to avoid fasting, but it seems to me that the same reasoning of R' Shlomo Zalman Auerbach, ztz"l applies here just as well.
Interestingly, according to the reason of the Chasam Sofer, a non-Jewish בכור might also have a reason to fast. It stands to reason that those non-Jewish בכורים who wanted to attach themselves by conversion to Klal Yisrael would have also fasted. There are, in fact, poskim who recommend that a ger who was a בכור should try to attend a siyum. You know me -- give me an inch and I'll take a mile -- I plan to make my own siyum on Bava Kamma, just to cover all my bases.