"If its legal" is a bit of a misnomer in some cases, as different countries have varying laws and guidelines for (fictional) material. A good general rule of thumb however is to strictly avoid anything involving anyone under the age of 18, or anything that may appear to be under the age of 18 (so sexy time with a 100 year old vampire child still counts as a faux pas). You may also want to avoid extremely graphic and/or violent sexual content (IE: Explicit rape, sexual torture, etc).