Generally, any property acquired prior to the marriage is considered to be the separate property of that spouse, not subject to be divided during a subsequent divorce. However, to the extent that marital funds are used to pay down a debt associated with acquiring an interest in that property, it does become marital property subject to be divided. So, for example, if a home is purchased prior to the marriage, but the couple uses funds earned during the marriage to pay the mortgage, a portion of the equity in the home (if any) may be marital property. However, it only becomes marital property to be divided in the case of a divorce. If one spouse keeps title in his or her name, and then dies, the other spouse may not be able to claim an interest in the house. Also, if title was placed in both parties names prior to the marriage, then both spouses own it at the time title was changed. So, the answer is, no, they don't automatically become the property of both spouses, but they may over time or depending on how the property is titled. In any event, you should consult an attorney regarding your circumstances and consider entering into a prenuptial agreement to preserve your rights.
Separate (premarital) property won't "automatically" become Joint (marital) property after marriage. BUT, don't presume that separate property simply stays separate through marriage. Commingling, payment of debts, improvements, and/or joint use can convert separate property to joint property, and this is not an exhaustive list. You should presume that once you get married, all property eventually will become joint property unless you specifically make efforts to keep it separate.