Ibn Ezra on Leviticus 18:19

והשוכב עם הנדה חשוב מהעריות: One who sleeps with a menstruating woman is considered to have slept with a woman whose nakedness one is prohibited from uncovering.⁶⁴Even though the prohibition to have intercourse with her is limited to the time of her menses and the child born from such an act is not considered a mamzer (illegitimate).