Albert Barnes Commentary Exodus 21:3

"If he come in by himself, he shall go out by himself: if he be married, then his wife shall go out with him." — Exodus 21:3 (ASV)

If a married man became a bondservant, his rights concerning his wife were respected. However, if a single bondservant accepted a bondservant woman from his master as his wife, the master did not lose his claim to the woman or her children when the husband’s term of service ended. Presumably, such wives were always foreign slaves.