Actually, many guys experience erection and (possibly) ejaculation while working out. This is a completely normal thing to happen to a guy. It is because of the blood flow to the area can be altered by things like long situp routines and other ab crunchers that flush blood throughout your pelvic region.

It doesn't mean that you are gay, or are sexually excited by working out. Now, this does not mean that a pile of situps will make you ejaculate, as this only occurs in certain men. Many guys get embarrased about getting an erection at the gym or what not, but are curious as to what causes it.