Is masturbating unsafe? Will it cause harm later in marriage?
See "Is masturbation unacceptable?" for a discussion about whether it is right or wrong.
In regards to marriage, most men who use masturbation as way to curb their sexual desire find that it is no longer needed after marriage (I Corinthians 7:1-9), except in rare occasions when sex with one's spouse is not possible for an extended period.
Those who use it to stimulate their sexual desire, especially those who use pornography and unclean thoughts about women to inflame their desire, find it difficult to stop because of the bad habits they have created in themselves. Some find their imaginations more pleasant than reality and this harms the relationship between a husband and his wife. See "A Look at Pornography" in regards to the damage pornography can do to a person.