When do you think officers and enlisted can enter into a personal relationship?

Actually, In the Army, 600-20 governs the rare instances this is allowable. (They are Rare) I suspect the USN and all other branches have similar regulations. Since it appears you are training to be a Commissioned Officer I would offer that the first thing you need to know if you have a question is what is the standard (regulation, policy etc.). Second, you number one mission as an Officer (before mission accomplishment, taking care of Soldiers (Sailors), and improving your organization), is to model (live) your services values. This is what 600-20 (Army Command Policy) says:

(2) Dating, shared living accommodations other than those directed by operational requirements, and intimate or sexual relationships between officers and enlisted personnel, or NCOs and junior enlisted Soldiers. This prohibition does not apply to the following:

(a) When evidence of fraternization between an officer and enlisted member or an NCO and a junior enlisted Soldier prior to their marriage exists, their marriage does not preclude appropriate command action based on the prior fraternization. Commanders have a wide range of responses available including counseling, reprimand, order to cease, reassignment, administrative action, or adverse action. Commanders must carefully consider all of the facts and circumstances in reaching a disposition that is appropriate. Generally, the commander should take the minimum action necessary to ensure that the needs of good order and discipline are satisfied.

(b) Situations in which a relationship that complies with this policy would move into noncompliance due to a change in status of one of the members (for instance, a case where two junior enlisted members are dating and one is subsequently commissioned or selected to be a WO, commissioned officer, or NCO). In relationships where one of the
enlisted members has entered into a program intended to result in a change in his or her status from enlisted to officer or junior enlisted Soldier to NCO, the couple must terminate the relationship permanently or marry within one year of the date of the appointment or the change in status occurs.

(c) Personal relationships between members of the National Guard or Army Reserve, when the relationship primarily exists due to civilian acquaintanceships, unless the individuals are on AD (other than AT), on FTNGD (other than AT), or serving as a dual status military technician.

(d) Personal relationships between members of the RA and members of the National Guard or Army Reserve when the relationship primarily exists due to civilian association and the RC member is not on AD (other than AT), on FTNGD (other than AT), or serving as a dual status military technician.

(e) Prohibited relationships involving dual status military technicians, which were not prohibited under previously existing rules and regulations, are exempt until 1 March 2015.

(f) Soldiers and leaders share responsibility for ensuring that these personal relationships do not interfere with good order and discipline. Commanders will ensure that personal relationships that exist between Soldiers of different grades emanating from their civilian careers will not influence training, readiness, or personnel actions.

1) Pre-existing relationship. As an example. I had an active duty wife in the Band. Later in her Career she could have gone WO. Should she or I have to give up our respective careers? What if I had the opportunity to pursue a commission? Should she have to get out?

2) Different Services. Completely different chain of commands. It's not Frat.

3) Reserve / Active. Reservists are "civilians" most of the time, and trying to apply military logic to civilian relationships doesn't always work.

If it's not within the chain of command and it doesn't negatively affect mission, morale, or best interest, it shouldn't be a big deal. I have a very liberal mindset when it comes to social issues from within the military, ultimately I am all about the mission. Mission comes first before anything else, this be in corporate, or military life. If a relationship jeopardizes the mission in anyway, it needs to end.