Antimicrobial and contraceptive compositions and methods which prevent and/or reduce the risk of transmission of sexually transmitted diseases through sexual activity as well as prevent and/or reduce the risk of pregnancy are provided. The compositions contain (1) a matrix-forming agent, (2) a bio-adhesive agent, (3) a buffering agent, (4) optionally a humectant, (5) optionally a preservative, and (6) water; wherein the composition is suitable for application within the vagina; wherein the compositions forms a semisolid matrix on contact with ejaculate (thereby trapping ejaculated microbes and spermatozoa); wherein the composition causes hardening of cervical mucus (thereby decreasing the probability of sperm entry); wherein the composition forms a bio-adhesive layer over vaginal surfaces (thereby preventing or reducing the risk of contact of STD-causing microbes with the vaginal surfaces); wherein the composition maintains an acidic vaginal pH of less than about 5 in the presence of semen ejaculated from the male; and wherein the composition does not significantly impair the natural microbiological balance within the vagina. The antimicrobial and contraceptive compositions may also contain additional antimicrobial and/or contraceptive agents (e.g., nonoxynol-9, octoxynol-9, benzalkonium chloride, phosphorylated hesperidins, sulfonated hesperidins, polystyrene sulfonates, substituted benzenesulfonic acid formaldehyde co-polymers, H2SO4-modified mandelic acids, povidone iodine, itraconazole, ketoconazole, metronidazole, clotrimazole, fluconazole, teraconazole, miconazole, tinidazole, iconazole, chloramphenicol, nystatin, cyclopiroxolamine, and the like).