Their love-making became less and less frequent.是什么意思？

Their love-making became less and less frequent.

他们做爱的频率越来越低。

their

pron. 他（她，它）们的，在提及性别不详的人时，用以代替 him 或 her；

例句
Their permissiveness toward their children reflects the wild abandon of their own lives.
他们对孩子的放任折射出他们自身生活的放浪形骸。

became

v. 变为，成为（ become的过去式 ）；

例句
It became very clear that the incident was not just an aberration, it was not just a single incident.
事实已经很清楚，这件事不是偶发的，它并不是一起孤立的事件。

less

adj. 较少的（little的比较级），较小的，较次的，较劣的，较不重要的；adv. 较少地，较小地；prep. 减去，少掉，不足；n. 较少，较小；

例句
The system is failing most disastrously among less academic children.
这种制度对学习不太好的孩子效果最差。

and

conj. 和，与，而且，于是，然后，因此；

例句
I spend a lot on expensive jewelry and clothing...
我花很多钱购买昂贵的首饰和衣服。

frequent

adj. 频繁的，时常发生的，常见的，（脉搏等）急促的，快的；

例句
Eleanor would later blame her mother-in-law for her husband's frequent absences.
埃莉诺过后会因丈夫经常不在家而责怪婆婆。