Does consensual sex break the five precepts? Is it considered breaking the five precepts under these circumstances:


Both are in a relationship, unmarried and it's purely consensual.  
Both are single and it's purely consensual.
There are four conditions for wrong conduct in sexual pleasures.

i) There must be a man or woman with whom it is improper to
have sexual intercourse.

ii) There must be intention to have sexual intercourse with such a
person.

iii) Action must be taken to have such an intercourse.

iv) There must be enjoyment from contact of the sexual organs.

With reference to the first condition, there are twenty kinds of
women with whom men should have no sexual relations. They can
be divided into three groups, namely: women under the
guardianship of parents, family members, relatives and authorities
charged with their care; married or betrothed women; bhikkhunis
and religious women observing the Holy Life. For all women, a
man forbidden by tradition or under religious rules is prohibited
as a partner. For any unwilling partner who is drugged or forced to
have sexual intercourse under threat of violence or coercion,
conditions (ii) &amp; (iv) exclude them from violation of the precept.

Causes of Sexual Misconduct

The root cause of sexual misconduct is always greed or lust,
accompanied by delusion.
Factors affecting the Gravity of Sexual Misconduct
The gravity of the offence is determined by the degree of lust
motivating the action and the qualities of the person against whom
the transgression is committed. When the lust is very strong, even
incest and rape can be committed, the most serious being the rape of
a female Arahant.

So to answer your questions,
consensuality is actually a requirement to break the 3rd precept. If a girl gets raped, she's not breaking the precept, but the man is.

Case #1: Yes, if it violates the 1st condition

Case #2: Yes, if it violates the 1st condition

Whether it's a long term relationship or just a one night stand is irrelevant to violating the 3rd precept. Lust is the root cause in both cases.