Would you get married to someone who wasn't 100% sure he or she could be faithful to you? Would you ever marry someone who had the tendency to cheat when an opportunity presented itself (such as having a one night stand) because they lacked self-control?
If you want an open relationship with this person, in which you both can sleep with other people, then by all means get married. Otherwise, why would you even consider being with someone you know won't be faithful? Don't set yourself up for heartbreak.