Do not talk about problems you are having with your spouse with someone of the opposite sex.
Have a friend of you and your marriage of the same sex to go to when you need to talk about difficulties in your marriage. Someone who is your friend and a friend of your marriage will listen with compassion, help you take responsibility for your part of the marriage, and guide you in doing things that are good for your marriage even if you do not like it. This person is not afraid to tell you the problem lies with you.
When you speak to someone of the opposite sex about problems in your marriage you run the risk of getting needs met from the person you are talking to when those needs should be met by your spouse. The same sex friend will help you address getting your needs met with your spouse. The opposite sex friend could end up being part of the problem.
Will there automatically be more problems if you talk to an opposite sex friend? No. Do not take the chance though. Remember that when you are having problems you are more vulnerable and that caring ear from someone else may mean more to you then you intended and lead you where you had not planned. This is why we make a proactive plan up front.
Does this mean you cannot have an opposite sex therapist? No. Therapists are trained to draw boundaries in these situations. My personal preference though is that if it is a marital problem that the two of you go together. By going together you automatically have a check and balance and you are more likely to work out the issue between the two of you instead of getting sidetracked by possible feelings for someone else.
Staying faithful is essential to your REAL marriage.