The use of two different tenses (present and perfect) is because Paul is actually including two different (though related) topics, a specific situation with marriages and a general principle he is applying to the situation.
The verse is structured: Specific situation (perfect), Paul’s universal application (perfect), universal axiom (perfect).
The first part of the verse deals with the issue of unbelieving spouses wanting to end the marriage with the believing partner. Presumably, this situation was contemporary to Paul and his readers, which explains the use of the present tense for ‘χωρίζεται’ (leaves) and ‘χωριζέσθω’ (let him leave).
The second part of the verse switches from describing a present (to the original audience) situation, to a universal, guiding principle which Paul believes should be applied to the present situation. Paul describes his application first (“the brother or the sister is not under bondage…”), then the actual principle (“God has called us to peace”). ‘δεδούλωται’ (under bondage) and ‘κέκληκεν’ (has called) are both in the perfect.
Wallace says that generically the perfect “describes an event that, completed in the past…, has results existing in the present.” He goes on to explain that the focus of the perfect is not the past action, but the present consequences.1 Wallace lists several specific usages, including the “gnomic perfect” which seems appropriate here: “The perfect tense may be used with a gnomic force, to speak of a generic or proverbial occurrence.”2 Once example he cites is Rom 7:2, which covers a similar topic (i.e. being bound in marriage).
I believe the use of the perfect with ‘δεδούλωται’ (under bondage) is not to imply the marriage itself was never in force, but to emphasize that a believer is never bound in such a situation. Somewhat repetitive, but to directly address your questions:
The issue on this verse is wherever bondage refers to the marriage bondage or not.
Why in 1 Corinthians 7:15 Paul responds to the believing spouse by a perfect tense statement (is not under bondage) to a previous present tense action (if the unbeliever departs, let him depart)?
The use of the perfect emphasizes the universal applicability of his conclusion. Had Paul used the present tense, it could imply his reasoning only applies the specific situation at Corinth.
If Paul is referring to the marriage bondage, why then is he using the perfect tense?
Wouldn't that mean that the marriage IS NOT and HAS NOT BEEN in force?
“Not under bondage” is not referencing to the marriage bond directly in the sense your questions implies. Instead it applies to the believer’s ongoing commitment to the marriage bond, given the situation. The believer is no longer bound to what was previously a valid and binding marriage.
If the unbeliever is leaving now, why the marriage wouldn't be in force before he leaves?
Then, at which point of time the marriage has not been in force in the past (completed action) until the present (ongoing result of the completed action)?
The marriage was in force until the nonbeliever decided to leave. The force of the perfect here is not the past action, but the preset applicability. Further, it is not about entering the marriage bond, but about the requirement of a believer to be presently committed to the marriage. The completed action with ongoing result is the general principle that believers are not bound is such situations – so the perfect indicates they have never and continue not to be held is these types of cases.
1. Wallace, Daniel. Greek Grammar Beyond the Basics. Zondervan: Grand Rapids, Michigan. 1996. Page 573.
2. Ibid., page 580.