Strap-on harnesses are designed to hold a dildo upright and rigid against the pubic bone -- much like an erect penis.  This enables you to penetrate your partner while your hands are free to roam.  Although strap-ons have traditionally been popular in the lesbian and gay communities, they are quickly making their way into middle-American bedrooms.  In fact, many strap-ons are owned by straight women because more and more husbands and boyfriends want to enjoy the pleasures of prostate orgasms.