When some people use condoms they are able to enjoy sex without the worry of pregnancy or catching diseases and some people think since they would not have to worry about those things it will be more happening a lot more regardless if people are married or not and maybe that is what might be considered immoral.
Condom-use does not violate the principles of sexual morality. The sexual act is not representative of morality unless performed outside of marriage. The condom major purpose is to prevent pregnancy. Whether the couple is married or not would present a major moral issue. The moral issue is the partners' relationship.