"It is only for women"

Joseph Pilates was a boxer and trained boxers, and the method was used in male athletic and military conditioning long before it reached commercial fitness. Its current demographic skew is a marketing artefact rather than anything about the method.

Strong lifters routinely find one-spring arm work difficult, because the demand is control rather than force.

"It is easy" and "it is just stretching"

Both come from watching rather than doing. Pilates looks undramatic because the movements are slow and small; the effort is isometric and stabilizing, which is invisible from outside.

Stretching is a minor component. Most of a session is controlled loading against resistance.

"It lengthens muscles"

Muscle length is determined by attachment points, and no exercise changes those. What improves is the range your nervous system permits — a real effect, described inaccurately.

The "long lean muscle" claim is marketing. Muscle shape is determined by genetics, and size by loading and nutrition.

Myth and reality
MythReality
Only for womenDeveloped by and for athletes of both sexes
It is easyIsometric and stabilizing effort is invisible, not absent
Just stretchingMostly controlled loading against resistance
Lengthens musclesImproves nervous-system-permitted range
Need to be flexibleFlexibility is an outcome, not a prerequisite
Replaces strength trainingLoad increments are too coarse for that

"You need to be flexible to start"

This one keeps people away who would benefit most. Flexibility is an outcome of the practice, not a prerequisite for it, and the reformer in particular assists movements you cannot yet achieve unaided.

The people who find the first month hardest are usually the ones who benefit most over the following year.