Does breastmilk cause an infant to be constipated?

No. Breast milk will not cause constipation. The frequency of bowel movements will decrease as your child ages. As long as your child is not having hard, painful stools, then there is no constipation.
Define constipation. Your typical breast fed baby can have a little poo smear every feeding or one giant blowout every 7-10 days. If soft, i don't consider it constipation. Wetting every 6 hours or more & feeding vigorously are signs of good basic health. All babies pull legs & arms in when fussing as a reflex.Grunting is normal, they have no social graces.
No. Breastfed babies tend to have very soft stools. However, it is important to distinguish between constipation (hard stools) and stool infrequency (having a stool every couple of days). They are not the same thing.