1. Pastoral Counseling Center of Southern Maryland (
http://www.pccstmary.org/; 301-863-9333; in Lexington Park on Great Mills Rd.). I believe the two lead counselors (Arthur & Betty Joanne Scott) are the only two (or two of very, very few) MFT (Marriage and Family Therapy) licensed counselors in the county (though a Christian counseling center, like all reputable counseling centers, they counsel from the perspective of the client (in other words, if you want a Christian angle they will include it, if you don't they don't).
2. Jessica Bowling (office in Hollywood, MD; 301-752-2615;
http://www.jessicazbowlinglcpc.com/) is a licensed Professional Counselor (LCPC) who works with marriages, families, kids, etc.
3. I've also heard some good things about Vickie Elsesser-Vu (301-904-0909; she's a Licensed Clinical Social Worker (LCSW-C in MD) up in Mechanicsville, but I think she also works with couples).