"active in a relationship" is a correct and usable phrase in written English.
You can use it to describe a situation when two people are involved in a romantic relationship. For example, "The couple have been active in their relationship for over a decade."
He has been sexually active in a monogamous relationship.
"If you don't go to parties people assume you aren't sexually active unless you're in a relationship.
We found that 51.6% of the sexually active females had been in a relationship for more than a year; the corresponding figure for males was 41.4%.
Five of the eight mentees also reported active participation in a mentoring relationship specifically fostered a positive attitude and perspective toward their major.
Medicine was primarily an art form, in which the care, concern, and compassion of the physician were the most active ingredients in establishing a relationship of care and concern and healing patients.
This doesn't mean that she has to be super active in the relationship.
In late 2013 Daley became one of only a handful of active sportsmen to reveal he was in a relationship with another man.
Whereas a tutor, teacher/educator, coach, or supervisor mainly focuses on promoting and supporting a junior's professional skills, a mentor is an active partner in an ongoing relationship who helps a mentee to maximize his or her potential and to reach personal and professional goals [ 4].
Once you're in a healthy, active, mutually-respecting relationship, you'll begin to realize that all the things you thought you needed in a relationship were, in fact, things that kept you from achieving your fullest happiness.
The PCs demonstrated a good awareness of this area and were active in developing relationships with relevant agencies.
